Pi - wikipedia Pi This article is about the mathematical constant. For the Greek letter, see Pi ( letter ). For other uses, see Pi ( disambiguation ). Part of a series of articles on the mathematical constant π 3.14159 26535 89793 23846 26433... Uses Area of a circle Circumference Use in other formulae Properties Irrationality Transcendence Value Less than 22 / 7 Approximations Memorization People Archimedes Liu Hui Zu Chongzhi Aryabhata Madhava Ludolph van Ceulen Seki Takakazu Takebe Kenko William Jones John Machin William Shanks Srinivasa Ramanujan John Wrench Chudnovsky brothers Yasumasa Kanada History Chronology Book In culture Legislation Pi Day Related topics Squaring the circle Basel problem Six nines in π Other topics related to π The number π ( / paɪ / ) is a mathematical constant. Originally defined as the ratio of a circle 's circumference to its diameter, it now has various equivalent definitions and appears in many formulas in all areas of mathematics and physics. It is approximately equal to 3.14159. It has been represented by the Greek letter `` π '' since the mid-18th century, though it is also sometimes spelled out as `` pi ''. It is also called Archimedes ' constant. Being an irrational number, π can not be expressed exactly as a common fraction ( equivalently, its decimal representation never ends and never settles into a permanent repeating pattern ). Still, fractions such as 22 / 7 and other rational numbers are commonly used to approximate π. The digits appear to be randomly distributed. In particular, the digit sequence of π is conjectured to satisfy a specific kind of statistical randomness, but to date, no proof of this has been discovered. Also, π is a transcendental number ; that is, a number that is not the root of any non-zero polynomial having rational coefficients. This transcendence of π implies that it is impossible to solve the ancient challenge of squaring the circle with a compass and straightedge. Ancient civilizations required fairly accurate computed values for π for practical reasons, including the Egyptians and Babylonians. Around 250 BC the Greek mathematician Archimedes created an algorithm for calculating it. It was approximated to seven digits, using geometrical techniques, in Chinese mathematics, and to about five digits in Indian mathematics in the 5th century AD. The historically first exact formula for π, based on infinite series, was not available until a millennium later, when in the 14th century the Madhava -- Leibniz series was discovered in Indian mathematics. In the 20th and 21st centuries, mathematicians and computer scientists discovered new approaches that, when combined with increasing computational power, extended the decimal representation of π to many trillions of digits after the decimal point. Practically all scientific applications require no more than a few hundred digits of π, and many substantially fewer, so the primary motivation for these computations is the quest to find more efficient algorithms for calculating lengthy numeric series, as well as the desire to break records. The extensive calculations involved have also been used to test supercomputers and high - precision multiplication algorithms. Because its most elementary definition relates to the circle, π is found in many formulae in trigonometry and geometry, especially those concerning circles, ellipses, and spheres. In more modern mathematical analysis, the number is instead defined using the spectral properties of the real number system, as an eigenvalue or a period, without any reference to geometry. It appears therefore in areas of mathematics and the sciences having little to do with the geometry of circles, such as number theory and statistics, as well as in almost all areas of physics. The ubiquity of π makes it one of the most widely known mathematical constants both inside and outside the scientific community ; several books devoted to it have been published, the number is celebrated on Pi Day, and record - setting calculations of the digits of π often result in news headlines. Attempts to memorize the value of π with increasing precision have led to records of over 70,000 digits. Contents 1 Fundamentals 1.1 Name 1.2 Definition 1.3 Irrationality and normality 1.4 Transcendence 1.5 Continued fractions 1.6 Approximate value and digits 1.7 Complex numbers and Euler 's identity 2 History 2.1 Antiquity 2.2 Polygon approximation era 2.3 Infinite series 2.3. 1 Rate of convergence 2.4 Irrationality and transcendence 2.5 Adoption of the symbol π 3 Modern quest for more digits 3.1 Computer era and iterative algorithms 3.2 Motives for computing π 3.3 Rapidly convergent series 3.4 Monte Carlo methods 3.5 Spigot algorithms 4 Role and characterizations in mathematics 4.1 Geometry and trigonometry 4.2 Eigenvalues 4.3 Inequalities 4.4 Fourier transform and Heisenberg uncertainty principle 4.5 Gaussian integrals 4.6 Projective geometry 4.7 Topology 4.8 Vector calculus 4.9 Cauchy 's integral formula 4.10 The gamma function and Stirling 's approximation 4.11 Number theory and Riemann zeta function 4.12 Fourier series 4.13 Modular forms and theta functions 4.14 Cauchy distribution and potential theory 4.15 Complex dynamics 5 Outside mathematics 5.1 Describing physical phenomena 5.2 Memorizing digits 5.3 In popular culture 5.4 In computer culture 6 Notes 7 Further reading 8 External links Fundamentals Name The symbol used by mathematicians to represent the ratio of a circle 's circumference to its diameter is the lowercase Greek letter π, sometimes spelled out as pi, and derived from the first letter of the Greek word perimetros, meaning circumference. In English, π is pronounced as `` pie '' ( / paɪ /, paɪ ). In mathematical use, the lowercase letter π ( or π in sans - serif font ) is distinguished from its capitalized and enlarged counterpart ∏, which denotes a product of a sequence, analogous to how ∑ denotes summation. The choice of the symbol π is discussed in the section Adoption of the symbol π. Definition The circumference of a circle is slightly more than three times as long as its diameter. The exact ratio is called π. π is commonly defined as the ratio of a circle 's circumference C to its diameter d : π = C d ( displaystyle pi = ( \frac ( C ) ( d ) ) ) The ratio C / d is constant, regardless of the circle 's size. For example, if a circle has twice the diameter of another circle it will also have twice the circumference, preserving the ratio C / d. This definition of π implicitly makes use of flat ( Euclidean ) geometry ; although the notion of a circle can be extended to any curved ( non-Euclidean ) geometry, these new circles will no longer satisfy the formula π = C / d. Here, the circumference of a circle is the arc length around the perimeter of the circle, a quantity which can be formally defined independently of geometry using limits, a concept in calculus. For example, one may compute directly the arc length of the top half of the unit circle given in Cartesian coordinates by x + y = 1, as the integral : π = ∫ − 1 1 d x 1 − x 2. ( displaystyle pi = int _ ( - 1 ) ^ ( 1 ) ( \frac ( dx ) ( sqrt ( 1 - x ^ ( 2 ) ) ) ). ) An integral such as this was adopted as the definition of π by Karl Weierstrass, who defined it directly as an integral in 1841. Definitions of π such as these that rely on a notion of circumference, and hence implicitly on concepts of the integral calculus, are no longer common in the literature. Remmert ( 1991 ) explains that this is because in many modern treatments of calculus, differential calculus typically precedes integral calculus in the university curriculum, so it is desirable to have a definition of π that does not rely on the latter. One such definition, due to Richard Baltzer, and popularized by Edmund Landau, is the following : π is twice the smallest positive number at which the cosine function equals 0. The cosine can be defined independently of geometry as a power series, or as the solution of a differential equation. In a similar spirit, π can be defined instead using properties of the complex exponential, exp ( z ), of a complex variable z. Like the cosine, the complex exponential can be defined in one of several ways. The set of complex numbers at which exp ( z ) is equal to one is then an ( imaginary ) arithmetic progression of the form : (..., − 2 π i, 0, 2 π i, 4 π i,... ) = ( 2 π k i ∣ k ∈ Z ) ( displaystyle ( dots, - 2 pi i, 0, 2 pi i, 4 pi i, dots ) = ( 2 pi ki mid k in mathbb ( Z ) ) ) and there is a unique positive real number π with this property. A more abstract variation on the same idea, making use of sophisticated mathematical concepts of topology and algebra, is the following theorem : there is a unique ( up to automorphism ) continuous isomorphism from the group R / Z of real numbers under addition modulo integers ( the circle group ) onto the multiplicative group of complex numbers of absolute value one. The number π is then defined as half the magnitude of the derivative of this homomorphism. A circle encloses the largest area that can be attained within a given perimeter. Thus the number π is also characterized as the best constant in the isoperimetric inequality ( times one - fourth ). There are many other, closely related, ways in which π appears as an eigenvalue of some geometrical or physical process ; see below. Irrationality and normality π is an irrational number, meaning that it can not be written as the ratio of two integers ( fractions such as 22 / 7 are commonly used to approximate π, but no common fraction ( ratio of whole numbers ) can be its exact value ). Because π is irrational, it has an infinite number of digits in its decimal representation, and it does not settle into an infinitely repeating pattern of digits. There are several proofs that π is irrational ; they generally require calculus and rely on the reductio ad absurdum technique. The degree to which π can be approximated by rational numbers ( called the irrationality measure ) is not precisely known ; estimates have established that the irrationality measure is larger than the measure of e or ln ( 2 ) but smaller than the measure of Liouville numbers. The digits of π have no apparent pattern and have passed tests for statistical randomness, including tests for normality ; a number of infinite length is called normal when all possible sequences of digits ( of any given length ) appear equally often. The conjecture that π is normal has not been proven or disproven. Since the advent of computers, a large number of digits of π have been available on which to perform statistical analysis. Yasumasa Kanada has performed detailed statistical analyses on the decimal digits of π and found them consistent with normality ; for example, the frequencies of the ten digits 0 to 9 were subjected to statistical significance tests, and no evidence of a pattern was found. Any random sequence of digits contains arbitrarily long subsequences that appear non-random, by the infinite monkey theorem. Thus, because the sequence of π 's digits passes statistical tests for randomness, it contains some sequences of digits that may appear non-random, such as a sequence of six consecutive 9s that begins at the 762nd decimal place of the decimal representation of π. This is also called the `` Feynman point '' in mathematical folklore, after Richard Feynman, although no connection to Feynman is known. Transcendence Because π is a transcendental number, squaring the circle is not possible in a finite number of steps using the classical tools of compass and straightedge. In addition to being irrational, more strongly π is a transcendental number, which means that it is not the solution of any non-constant polynomial equation with rational coefficients, such as x / 120 − x / 6 + x = 0. The transcendence of π has two important consequences : First, π can not be expressed using any finite combination of rational numbers and square roots or n - th roots such as √ 31 or √ 10. Second, since no transcendental number can be constructed with compass and straightedge, it is not possible to `` square the circle ''. In other words, it is impossible to construct, using compass and straightedge alone, a square whose area is exactly equal to the area of a given circle. Squaring a circle was one of the important geometry problems of the classical antiquity. Amateur mathematicians in modern times have sometimes attempted to square the circle and sometimes claim success despite the fact that it is mathematically impossible. Continued fractions The constant π is represented in this mosaic outside the Mathematics Building at the Technical University of Berlin. Like all irrational numbers, π can not be represented as a common fraction ( also known as a simple or vulgar fraction ), by the very definition of `` irrational number '' ( that is, `` not a rational number '' ). But every irrational number, including π, can be represented by an infinite series of nested fractions, called a continued fraction : π = 3 + 1 7 + 1 15 + 1 1 + 1 292 + 1 1 + 1 1 + 1 1 + ⋱ ( displaystyle pi = 3 + \textstyle ( cfrac ( 1 ) ( 7 + \textstyle ( cfrac ( 1 ) ( 15 + \textstyle ( cfrac ( 1 ) ( 1 + \textstyle ( cfrac ( 1 ) ( 292 + \textstyle ( cfrac ( 1 ) ( 1 + \textstyle ( cfrac ( 1 ) ( 1 + \textstyle ( cfrac ( 1 ) ( 1 + ddots ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) Truncating the continued fraction at any point yields a rational approximation for π ; the first four of these are 3, 22 / 7, 333 / 106, and 355 / 113. These numbers are among the most well - known and widely used historical approximations of the constant. Each approximation generated in this way is a best rational approximation ; that is, each is closer to π than any other fraction with the same or a smaller denominator. Because π is known to be transcendental, it is by definition not algebraic and so can not be a quadratic irrational. Therefore, π can not have a periodic continued fraction. Although the simple continued fraction for π ( shown above ) also does not exhibit any other obvious pattern, mathematicians have discovered several generalized continued fractions that do, such as : π = 4 1 + 1 2 2 + 3 2 2 + 5 2 2 + 7 2 2 + 9 2 2 + ⋱ = 3 + 1 2 6 + 3 2 6 + 5 2 6 + 7 2 6 + 9 2 6 + ⋱ = 4 1 + 1 2 3 + 2 2 5 + 3 2 7 + 4 2 9 + ⋱ ( displaystyle ( \begin ( aligned ) pi & = \textstyle ( cfrac ( 4 ) ( 1 + \textstyle ( cfrac ( 1 ^ ( 2 ) ) ( 2 + \textstyle ( cfrac ( 3 ^ ( 2 ) ) ( 2 + \textstyle ( cfrac ( 5 ^ ( 2 ) ) ( 2 + \textstyle ( cfrac ( 7 ^ ( 2 ) ) ( 2 + \textstyle ( cfrac ( 9 ^ ( 2 ) ) ( 2 + ddots ) ) ) ) ) ) ) ) ) ) ) ) = 3 + \textstyle ( cfrac ( 1 ^ ( 2 ) ) ( 6 + \textstyle ( cfrac ( 3 ^ ( 2 ) ) ( 6 + \textstyle ( cfrac ( 5 ^ ( 2 ) ) ( 6 + \textstyle ( cfrac ( 7 ^ ( 2 ) ) ( 6 + \textstyle ( cfrac ( 9 ^ ( 2 ) ) ( 6 + ddots ) ) ) ) ) ) ) ) ) ) ( 8pt ) & = \textstyle ( cfrac ( 4 ) ( 1 + \textstyle ( cfrac ( 1 ^ ( 2 ) ) ( 3 + \textstyle ( cfrac ( 2 ^ ( 2 ) ) ( 5 + \textstyle ( cfrac ( 3 ^ ( 2 ) ) ( 7 + \textstyle ( cfrac ( 4 ^ ( 2 ) ) ( 9 + ddots ) ) ) ) ) ) ) ) ) ) end ( aligned ) ) ) Approximate value and digits Some approximations of pi include : Integers : 3 Fractions : Approximate fractions include ( in order of increasing accuracy ) 22 / 7, 333 / 106, 355 / 113, 52163 / 16604, 103993 / 33102, and 245850922 / 78256779. ( List is selected terms from A063674 and A063673. ) Digits : The first 50 decimal digits are 3.14159 26535 89793 23846 26433 83279 50288 41971 69399 37510... ( see A000796 ) Digits in other number systems The first 48 binary ( base 2 ) digits ( called bits ) are 11.0010 0100 0011 1111 0110 1010 1000 1000 1000 0101 1010 0011... ( see A004601 ) The first 20 digits in hexadecimal ( base 16 ) are 3.243 F6A8885A308D31319... ( see A062964 ) The first five sexagesimal ( base 60 ) digits are 3 ; 8, 29, 44, 0, 47 ( see A060707 ) Complex numbers and Euler 's identity The association between imaginary powers of the number e and points on the unit circle centered at the origin in the complex plane given by Euler 's formula. Any complex number, say z, can be expressed using a pair of real numbers. In the polar coordinate system, one number ( radius or r ) is used to represent z 's distance from the origin of the complex plane and the other ( angle or φ ) to represent a counter-clockwise rotation from the positive real line as follows : z = r ⋅ ( cos ⁡ φ + i sin ⁡ φ ), ( displaystyle z = r cdot ( cos \varphi + i sin \varphi ), ) where i is the imaginary unit satisfying i = − 1. The frequent appearance of π in complex analysis can be related to the behavior of the exponential function of a complex variable, described by Euler 's formula : e i φ = cos ⁡ φ + i sin ⁡ φ, ( displaystyle e ^ ( i \varphi ) = cos \varphi + i sin \varphi, ) where the constant e is the base of the natural logarithm. This formula establishes a correspondence between imaginary powers of e and points on the unit circle centered at the origin of the complex plane. Setting φ = π in Euler 's formula results in Euler 's identity, celebrated by mathematicians because it contains the five most important mathematical constants : e i π + 1 = 0. ( displaystyle e ^ ( i pi ) + 1 = 0. ) There are n different complex numbers z satisfying z = 1, and these are called the `` n - th roots of unity ''. They are given by this formula : e 2 π i k / n ( k = 0, 1, 2,..., n − 1 ). ( displaystyle e ^ ( 2 pi ik / n ) qquad ( k = 0, 1, 2, dots, n - 1 ). ) History Main article : Approximations of π See also : Chronology of computation of π Antiquity The best - known approximations to π dating before the Common Era were accurate to two decimal places ; this was improved upon in Chinese mathematics in particular by the mid-first millennium, to an accuracy of seven decimal places. After this, no further progress was made until the late medieval period. Some Egyptologists have claimed that the ancient Egyptians used an approximation of π as 22 / 7 from as early as the Old Kingdom. This claim has met with skepticism. The earliest written approximations of π are found in Egypt and Babylon, both within one percent of the true value. In Babylon, a clay tablet dated 1900 -- 1600 BC has a geometrical statement that, by implication, treats π as 25 / 8 = 3.125. In Egypt, the Rhind Papyrus, dated around 1650 BC but copied from a document dated to 1850 BC, has a formula for the area of a circle that treats π as ( 16 / 9 ) ≈ 3.1605. Astronomical calculations in the Shatapatha Brahmana ( ca. 4th century BC ) use a fractional approximation of 339 / 108 ≈ 3.139 ( an accuracy of 9 × 10 ). Other Indian sources by about 150 BC treat π as √ 10 ≈ 3.1622. Polygon approximation Era π can be estimated by computing the perimeters of circumscribed and inscribed polygons. The first recorded algorithm for rigorously calculating the value of π was a geometrical approach using polygons, devised around 250 BC by the Greek mathematician Archimedes. This polygonal algorithm dominated for over 1,000 years, and as a result π is sometimes referred to as `` Archimedes ' constant ''. Archimedes computed upper and lower bounds of π by drawing a regular hexagon inside and outside a circle, and successively doubling the number of sides until he reached a 96 - sided regular polygon. By calculating the perimeters of these polygons, he proved that 223 / 71 < π < 22 / 7 ( that is 3.1408 < π < 3.1429 ). Archimedes ' upper bound of 22 / 7 may have led to a widespread popular belief that π is equal to 22 / 7. Around 150 AD, Greek - Roman scientist Ptolemy, in his Almagest, gave a value for π of 3.1416, which he may have obtained from Archimedes or from Apollonius of Perga. Mathematicians using polygonal algorithms reached 39 digits of π in 1630, a record only broken in 1699 when infinite series were used to reach 71 digits. Archimedes developed the polygonal approach to approximating π. In ancient China, values for π included 3.1547 ( around 1 AD ), √ 10 ( 100 AD, approximately 3.1623 ), and 142 / 45 ( 3rd century, approximately 3.1556 ). Around 265 AD, the Wei Kingdom mathematician Liu Hui created a polygon - based iterative algorithm and used it with a 3,072 - sided polygon to obtain a value of π of 3.1416. Liu later invented a faster method of calculating π and obtained a value of 3.14 with a 96 - sided polygon, by taking advantage of the fact that the differences in area of successive polygons form a geometric series with a factor of 4. The Chinese mathematician Zu Chongzhi, around 480 AD, calculated that π ≈ 355 / 113 ( a fraction that goes by the name Milü in Chinese ), using Liu Hui 's algorithm applied to a 12,288 - sided polygon. With a correct value for its seven first decimal digits, this value of 3.141592920... remained the most accurate approximation of π available for the next 800 years. The Indian astronomer Aryabhata used a value of 3.1416 in his Āryabhaṭīya ( 499 AD ). Fibonacci in c. 1220 computed 3.1418 using a polygonal method, independent of Archimedes. Italian author Dante apparently employed the value 3 + √ 2 / 10 ≈ 3.14142. The Persian astronomer Jamshīd al - Kāshī produced 9 sexagesimal digits, roughly the equivalent of 16 decimal digits, in 1424 using a polygon with 3 × 2 sides, which stood as the world record for about 180 years. French mathematician François Viète in 1579 achieved 9 digits with a polygon of 3 × 2 sides. Flemish mathematician Adriaan van Roomen arrived at 15 decimal places in 1593. In 1596, Dutch mathematician Ludolph van Ceulen reached 20 digits, a record he later increased to 35 digits ( as a result, π was called the `` Ludolphian number '' in Germany until the early 20th century ). Dutch scientist Willebrord Snellius reached 34 digits in 1621, and Austrian astronomer Christoph Grienberger arrived at 38 digits in 1630 using 10 sides, which remains the most accurate approximation manually achieved using polygonal algorithms. Infinite series Comparison of the convergence of several historical infinite series for π. S is the approximation after taking n terms. Each subsequent subplot magnifies the shaded area horizontally by 10 times. ( click for detail ) The calculation of π was revolutionized by the development of infinite series techniques in the 16th and 17th centuries. An infinite series is the sum of the terms of an infinite sequence. Infinite series allowed mathematicians to compute π with much greater precision than Archimedes and others who used geometrical techniques. Although infinite series were exploited for π most notably by European mathematicians such as James Gregory and Gottfried Wilhelm Leibniz, the approach was first discovered in India sometime between 1400 and 1500 AD. The first written description of an infinite series that could be used to compute π was laid out in Sanskrit verse by Indian astronomer Nilakantha Somayaji in his Tantrasamgraha, around 1500 AD. The series are presented without proof, but proofs are presented in a later Indian work, Yuktibhāṣā, from around 1530 AD. Nilakantha attributes the series to an earlier Indian mathematician, Madhava of Sangamagrama, who lived c. 1350 -- c. 1425. Several infinite series are described, including series for sine, tangent, and cosine, which are now referred to as the Madhava series or Gregory -- Leibniz series. Madhava used infinite series to estimate π to 11 digits around 1400, but that value was improved on around 1430 by the Persian mathematician Jamshīd al - Kāshī, using a polygonal algorithm. Isaac Newton used infinite series to compute π to 15 digits, later writing `` I am ashamed to tell you to how many figures I carried these computations ''. The first infinite sequence discovered in Europe was an infinite product ( rather than an infinite sum, which are more typically used in π calculations ) found by French mathematician François Viète in 1593 : 2 π = 2 2 ⋅ 2 + 2 2 ⋅ 2 + 2 + 2 2 ⋯ ( displaystyle ( \frac ( 2 ) ( pi ) ) = ( \frac ( sqrt ( 2 ) ) ( 2 ) ) cdot ( \frac ( sqrt ( 2 + ( sqrt ( 2 ) ) ) ) ( 2 ) ) cdot ( \frac ( sqrt ( 2 + ( sqrt ( 2 + ( sqrt ( 2 ) ) ) ) ) ) ( 2 ) ) cdots ) The second infinite sequence found in Europe, by John Wallis in 1655, was also an infinite product : π 2 = 2 1 ⋅ 2 3 ⋅ 4 3 ⋅ 4 5 ⋅ 6 5 ⋅ 6 7 ⋅ 8 7 ⋅ 8 9 ⋯ ( displaystyle ( \frac ( pi ) ( 2 ) ) = ( \frac ( 2 ) ( 1 ) ) cdot ( \frac ( 2 ) ( 3 ) ) cdot ( \frac ( 4 ) ( 3 ) ) cdot ( \frac ( 4 ) ( 5 ) ) cdot ( \frac ( 6 ) ( 5 ) ) cdot ( \frac ( 6 ) ( 7 ) ) cdot ( \frac ( 8 ) ( 7 ) ) cdot ( \frac ( 8 ) ( 9 ) ) cdots ) The discovery of calculus, by English scientist Isaac Newton and German mathematician Gottfried Wilhelm Leibniz in the 1660s, led to the development of many infinite series for approximating π. Newton himself used an arcsin series to compute a 15 digit approximation of π in 1665 or 1666, later writing `` I am ashamed to tell you to how many figures I carried these computations, having no other business at the time. '' In Europe, Madhava 's formula was rediscovered by Scottish mathematician James Gregory in 1671, and by Leibniz in 1674 : arctan ⁡ z = z − z 3 3 + z 5 5 − z 7 7 + ⋯ ( displaystyle arctan z = z - ( \frac ( z ^ ( 3 ) ) ( 3 ) ) + ( \frac ( z ^ ( 5 ) ) ( 5 ) ) - ( \frac ( z ^ ( 7 ) ) ( 7 ) ) + cdots ) This formula, the Gregory -- Leibniz series, equals π / 4 when evaluated with z = 1. In 1699, English mathematician Abraham Sharp used the Gregory -- Leibniz series for z = 1 3 ( displaystyle z = ( \frac ( 1 ) ( sqrt ( 3 ) ) ) ) to compute π to 71 digits, breaking the previous record of 39 digits, which was set with a polygonal algorithm. The Gregory -- Leibniz for z = 1 ( displaystyle z = 1 ) series is simple, but converges very slowly ( that is, approaches the answer gradually ), so it is not used in modern π calculations. In 1706 John Machin used the Gregory -- Leibniz series to produce an algorithm that converged much faster : π 4 = 4 arctan ⁡ 1 5 − arctan ⁡ 1 239 ( displaystyle ( \frac ( pi ) ( 4 ) ) = 4 , arctan ( \frac ( 1 ) ( 5 ) ) - arctan ( \frac ( 1 ) ( 239 ) ) ) Machin reached 100 digits of π with this formula. Other mathematicians created variants, now known as Machin - like formulae, that were used to set several successive records for calculating digits of π. Machin - like formulae remained the best - known method for calculating π well into the age of computers, and were used to set records for 250 years, culminating in a 620 - digit approximation in 1946 by Daniel Ferguson -- the best approximation achieved without the aid of a calculating device. A record was set by the calculating prodigy Zacharias Dase, who in 1844 employed a Machin - like formula to calculate 200 decimals of π in his head at the behest of German mathematician Carl Friedrich Gauss. British mathematician William Shanks famously took 15 years to calculate π to 707 digits, but made a mistake in the 528th digit, rendering all subsequent digits incorrect. Rate of convergence Some infinite series for π converge faster than others. Given the choice of two infinite series for π, mathematicians will generally use the one that converges more rapidly because faster convergence reduces the amount of computation needed to calculate π to any given accuracy. A simple infinite series for π is the Gregory -- Leibniz series : π = 4 1 − 4 3 + 4 5 − 4 7 + 4 9 − 4 11 + 4 13 − ⋯ ( displaystyle pi = ( \frac ( 4 ) ( 1 ) ) - ( \frac ( 4 ) ( 3 ) ) + ( \frac ( 4 ) ( 5 ) ) - ( \frac ( 4 ) ( 7 ) ) + ( \frac ( 4 ) ( 9 ) ) - ( \frac ( 4 ) ( 11 ) ) + ( \frac ( 4 ) ( 13 ) ) - cdots ) As individual terms of this infinite series are added to the sum, the total gradually gets closer to π, and -- with a sufficient number of terms -- can get as close to π as desired. It converges quite slowly, though -- after 500,000 terms, it produces only five correct decimal digits of π. An infinite series for π ( published by Nilakantha in the 15th century ) that converges more rapidly than the Gregory -- Leibniz series is : π = 3 + 4 2 × 3 × 4 − 4 4 × 5 × 6 + 4 6 × 7 × 8 − 4 8 × 9 × 10 + ⋯ ( displaystyle pi = 3 + ( \frac ( 4 ) ( 2 \times 3 \times 4 ) ) - ( \frac ( 4 ) ( 4 \times 5 \times 6 ) ) + ( \frac ( 4 ) ( 6 \times 7 \times 8 ) ) - ( \frac ( 4 ) ( 8 \times 9 \times 10 ) ) + cdots ) The following table compares the convergence rates of these two series : Infinite series for π After 1st term After 2nd term After 3rd term After 4th term After 5th term Converges to : π = 4 1 − 4 3 + 4 5 − 4 7 + 4 9 − 4 11 + 4 13 ⋯. ( displaystyle pi = ( \frac ( 4 ) ( 1 ) ) - ( \frac ( 4 ) ( 3 ) ) + ( \frac ( 4 ) ( 5 ) ) - ( \frac ( 4 ) ( 7 ) ) + ( \frac ( 4 ) ( 9 ) ) - ( \frac ( 4 ) ( 11 ) ) + ( \frac ( 4 ) ( 13 ) ) cdots. ) 4.0000 2.6666... 3.4666... 2.8952... 3.3396... π = 3.1415... π = 3 + 4 2 × 3 × 4 − 4 4 × 5 × 6 + 4 6 × 7 × 8 ⋯. ( displaystyle pi = ( 3 ) + ( \frac ( 4 ) ( 2 \times 3 \times 4 ) ) - ( \frac ( 4 ) ( 4 \times 5 \times 6 ) ) + ( \frac ( 4 ) ( 6 \times 7 \times 8 ) ) cdots. ) 3.0000 3.1666... 3.1333... 3.1452... 3.1396... After five terms, the sum of the Gregory -- Leibniz series is within 0.2 of the correct value of π, whereas the sum of Nilakantha 's series is within 0.002 of the correct value of π. Nilakantha 's series converges faster and is more useful for computing digits of π. Series that converge even faster include Machin 's series and Chudnovsky 's series, the latter producing 14 correct decimal digits per term. Irrationality and transcendence See also : Proof that π is irrational and Proof that π is transcendental Not all mathematical advances relating to π were aimed at increasing the accuracy of approximations. When Euler solved the Basel problem in 1735, finding the exact value of the sum of the reciprocal squares, he established a connection between π and the prime numbers that later contributed to the development and study of the Riemann zeta function : π 2 6 = 1 1 2 + 1 2 2 + 1 3 2 + 1 4 2 + ⋯ ( displaystyle ( \frac ( pi ^ ( 2 ) ) ( 6 ) ) = ( \frac ( 1 ) ( 1 ^ ( 2 ) ) ) + ( \frac ( 1 ) ( 2 ^ ( 2 ) ) ) + ( \frac ( 1 ) ( 3 ^ ( 2 ) ) ) + ( \frac ( 1 ) ( 4 ^ ( 2 ) ) ) + cdots ) Swiss scientist Johann Heinrich Lambert in 1761 proved that π is irrational, meaning it is not equal to the quotient of any two whole numbers. Lambert 's proof exploited a continued - fraction representation of the tangent function. French mathematician Adrien - Marie Legendre proved in 1794 that π is also irrational. In 1882, German mathematician Ferdinand von Lindemann proved that π is transcendental, confirming a conjecture made by both Legendre and Euler. Hardy and Wright states that `` the proofs were afterwards modified and simplified by Hilbert, Hurwitz, and other writers ''. Adoption of the symbol π Leonhard Euler popularized the use of the Greek letter π in works he published in 1736 and 1748. In the earliest usages, the Greek letter π was an abbreviation of the Greek word for periphery ( περιφέρεια ), and was combined in ratios with δ ( for diameter ) or ρ ( for radius ) to form circle constants. ( Before then, mathematicians sometimes used letters such as c or p instead. ) The first recorded use is Oughtred 's `` δ. π ( displaystyle delta. pi ) ``, to express the ratio of periphery and diameter in the 1647 and later editions of Clavis Mathematicae. Barrow likewise used '' π δ ( \textstyle ( \frac ( pi ) ( delta ) ) ) `` to represent the constant 3.14..., while Gregory instead used '' π ρ ( \textstyle ( \frac ( pi ) ( \rho ) ) ) `` to represent 6.28.... The earliest known use of the Greek letter π alone to represent the ratio of a circle 's circumference to its diameter was by Welsh mathematician William Jones in his 1706 work Synopsis Palmariorum Matheseos ; or, a New Introduction to the Mathematics. The Greek letter first appears there in the phrase `` 1 / 2 Periphery ( π ) '' in the discussion of a circle with radius one. However, he writes that his equations for π are from the `` ready pen of the truly ingenious Mr. John Machin '', leading to speculation that Machin may have employed the Greek letter before Jones. Jones ' notation was not immediately adopted by other mathematicians, with the fraction notation still being used as late as 1767. Euler started using the single - letter form beginning with his 1727 Essay Explaining The Properties Of Air, though he used π = 6.28..., the ratio of radius to periphery, in this and some later writing. Euler first used π = 3.14... in his 1736 work Mechanica, and continued in his widely - read 1748 work Introductio in analysin infinitorum ( he wrote : `` for the sake of brevity we will write this number as π ; thus π is equal to half the circumference of a circle of radius 1 '' ). Because Euler corresponded heavily with other mathematicians in Europe, the use of the Greek letter spread rapidly, and the practice was universally adopted thereafter in the Western world. Modern quest for more digits Computer Era and iterative algorithms John von Neumann was part of the team that first used a digital computer, ENIAC, to compute π. The Gauss -- Legendre iterative algorithm : Initialize a 0 = 1 b 0 = 1 2 t 0 = 1 4 p 0 = 1 ( displaystyle scriptstyle a_ ( 0 ) = 1 quad b_ ( 0 ) = ( \frac ( 1 ) ( sqrt ( 2 ) ) ) quad t_ ( 0 ) = ( \frac ( 1 ) ( 4 ) ) quad p_ ( 0 ) = 1 ) Iterate a n + 1 = a n + b n 2 b n + 1 = a n b n ( displaystyle scriptstyle a_ ( n + 1 ) = ( \frac ( a_ ( n ) + b_ ( n ) ) ( 2 ) ) quad quad b_ ( n + 1 ) = ( sqrt ( a_ ( n ) b_ ( n ) ) ) ) t n + 1 = t n − p n ( a n − a n + 1 ) 2 p n + 1 = 2 p n ( displaystyle scriptstyle t_ ( n + 1 ) = t_ ( n ) - p_ ( n ) ( a_ ( n ) - a_ ( n + 1 ) ) ^ ( 2 ) quad quad p_ ( n + 1 ) = 2p_ ( n ) ) Then an estimate for π is given by π ≈ ( a n + b n ) 2 4 t n ( displaystyle scriptstyle pi approx ( \frac ( ( a_ ( n ) + b_ ( n ) ) ^ ( 2 ) ) ( 4t_ ( n ) ) ) ) The development of computers in the mid-20th century again revolutionized the hunt for digits of π. American mathematicians John Wrench and Levi Smith reached 1,120 digits in 1949 using a desk calculator. Using an inverse tangent ( arctan ) infinite series, a team led by George Reitwiesner and John von Neumann that same year achieved 2,037 digits with a calculation that took 70 hours of computer time on the ENIAC computer. The record, always relying on an arctan series, was broken repeatedly ( 7,480 digits in 1957 ; 10,000 digits in 1958 ; 100,000 digits in 1961 ) until 1 million digits were reached in 1973. Two additional developments around 1980 once again accelerated the ability to compute π. First, the discovery of new iterative algorithms for computing π, which were much faster than the infinite series ; and second, the invention of fast multiplication algorithms that could multiply large numbers very rapidly. Such algorithms are particularly important in modern π computations because most of the computer 's time is devoted to multiplication. They include the Karatsuba algorithm, Toom -- Cook multiplication, and Fourier transform - based methods. The iterative algorithms were independently published in 1975 -- 1976 by American physicist Eugene Salamin and Australian scientist Richard Brent. These avoid reliance on infinite series. An iterative algorithm repeats a specific calculation, each iteration using the outputs from prior steps as its inputs, and produces a result in each step that converges to the desired value. The approach was actually invented over 160 years earlier by Carl Friedrich Gauss, in what is now termed the arithmetic -- geometric mean method ( AGM method ) or Gauss -- Legendre algorithm. As modified by Salamin and Brent, it is also referred to as the Brent -- Salamin algorithm. The iterative algorithms were widely used after 1980 because they are faster than infinite series algorithms : whereas infinite series typically increase the number of correct digits additively in successive terms, iterative algorithms generally multiply the number of correct digits at each step. For example, the Brent - Salamin algorithm doubles the number of digits in each iteration. In 1984, the Canadian brothers John and Peter Borwein produced an iterative algorithm that quadruples the number of digits in each step ; and in 1987, one that increases the number of digits five times in each step. Iterative methods were used by Japanese mathematician Yasumasa Kanada to set several records for computing π between 1995 and 2002. This rapid convergence comes at a price : the iterative algorithms require significantly more memory than infinite series. Motives for computing π As mathematicians discovered new algorithms, and computers became available, the number of known decimal digits of π increased dramatically. Note that the vertical scale is logarithmic. For most numerical calculations involving π, a handful of digits provide sufficient precision. According to Jörg Arndt and Christoph Haenel, thirty - nine digits are sufficient to perform most cosmological calculations, because that is the accuracy necessary to calculate the circumference of the observable universe with a precision of one atom. Accounting for additional digits needed to compensate for computational round - off errors, Arndt concludes that a few hundred digits would suffice for any scientific application. Despite this, people have worked strenuously to compute π to thousands and millions of digits. This effort may be partly ascribed to the human compulsion to break records, and such achievements with π often make headlines around the world. They also have practical benefits, such as testing supercomputers, testing numerical analysis algorithms ( including high - precision multiplication algorithms ) ; and within pure mathematics itself, providing data for evaluating the randomness of the digits of π. Rapidly convergent series Srinivasa Ramanujan, working in isolation in India, produced many innovative series for computing π. Modern π calculators do not use iterative algorithms exclusively. New infinite series were discovered in the 1980s and 1990s that are as fast as iterative algorithms, yet are simpler and less memory intensive. The fast iterative algorithms were anticipated in 1914, when the Indian mathematician Srinivasa Ramanujan published dozens of innovative new formulae for π, remarkable for their elegance, mathematical depth, and rapid convergence. One of his formulae, based on modular equations, is 1 π = 2 2 9801 ∑ k = 0 ∞ ( 4 k ) ! ( 1103 + 26390 k ) k ! 4 ( 396 4 k ). ( displaystyle ( \frac ( 1 ) ( pi ) ) = ( \frac ( 2 ( sqrt ( 2 ) ) ) ( 9801 ) ) sum _ ( k = 0 ) ^ ( infty ) ( \frac ( ( 4k ) ! ( 1103 + 26390k ) ) ( k ! ^ ( 4 ) ( 396 ^ ( 4k ) ) ) ). ) This series converges much more rapidly than most arctan series, including Machin 's formula. Bill Gosper was the first to use it for advances in the calculation of π, setting a record of 17 million digits in 1985. Ramanujan 's formulae anticipated the modern algorithms developed by the Borwein brothers and the Chudnovsky brothers. The Chudnovsky formula developed in 1987 is 1 π = 12 640320 3 / 2 ∑ k = 0 ∞ ( 6 k ) ! ( 13591409 + 545140134 k ) ( 3 k ) ! ( k ! ) 3 ( − 640320 ) 3 k. ( displaystyle ( \frac ( 1 ) ( pi ) ) = ( \frac ( 12 ) ( 640320 ^ ( 3 / 2 ) ) ) sum _ ( k = 0 ) ^ ( infty ) ( \frac ( ( 6k ) ! ( 13591409 + 545140134k ) ) ( ( 3k ) ! ( k ! ) ^ ( 3 ) ( - 640320 ) ^ ( 3k ) ) ). ) It produces about 14 digits of π per term, and has been used for several record - setting π calculations, including the first to surpass 1 billion ( 10 ) digits in 1989 by the Chudnovsky brothers, 2.7 trillion ( 2.7 × 10 ) digits by Fabrice Bellard in 2009, and 10 trillion ( 10 ) digits in 2011 by Alexander Yee and Shigeru Kondo. For similar formulas, see also the Ramanujan -- Sato series. In 2006, Canadian mathematician Simon Plouffe used the PSLQ integer relation algorithm to generate several new formulas for π, conforming to the following template : π k = ∑ n = 1 ∞ 1 n k ( a q n − 1 + b q 2 n − 1 + c q 4 n − 1 ), ( displaystyle pi ^ ( k ) = sum _ ( n = 1 ) ^ ( infty ) ( \frac ( 1 ) ( n ^ ( k ) ) ) left ( ( \frac ( a ) ( q ^ ( n ) - 1 ) ) + ( \frac ( b ) ( q ^ ( 2n ) - 1 ) ) + ( \frac ( c ) ( q ^ ( 4n ) - 1 ) ) \right ), ) where q is e ( Gelfond 's constant ), k is an odd number, and a, b, c are certain rational numbers that Plouffe computed. Monte Carlo methods Buffon 's needle. Needles a and b are dropped randomly. Random dots are placed on the quadrant of a square with a circle inscribed in it. Monte Carlo methods, based on random trials, can be used to approximate π. Monte Carlo methods, which evaluate the results of multiple random trials, can be used to create approximations of π. Buffon 's needle is one such technique : If a needle of length l is dropped n times on a surface on which parallel lines are drawn t units apart, and if x of those times it comes to rest crossing a line ( x > 0 ), then one may approximate π based on the counts : π ≈ 2 n l x t ( displaystyle pi approx ( \frac ( 2n ell ) ( xt ) ) ) Another Monte Carlo method for computing π is to draw a circle inscribed in a square, and randomly place dots in the square. The ratio of dots inside the circle to the total number of dots will approximately equal π / 4. Five random walks with 200 steps. The sample mean of W is μ = 56 / 5, and so 2 ( 200 ) μ ≈ 3.19 is within 0.05 of π Another way to calculate π using probability is to start with a random walk, generated by a sequence of ( fair ) coin tosses : independent random variables X such that X ∈ ( − 1, 1 ) with equal probabilities. The associated random walk is W n = ∑ k = 1 n X k ( displaystyle W_ ( n ) = sum _ ( k = 1 ) ^ ( n ) X_ ( k ) ) so that, for each n, W is drawn from a standard binomial distribution. As n varies W defines a ( discrete ) stochastic process. Then π can be calculated by π = lim n → ∞ 2 n E ( W n ) 2. ( displaystyle pi = lim _ ( n \to infty ) ( \frac ( 2n ) ( E ( W_ ( n ) ) ^ ( 2 ) ) ). ) This Monte Carlo method is independent of any relation to circles, and is a consequence of the central limit theorem, discussed above. These Monte Carlo methods for approximating π are very slow compared to other methods, and do not provide any information on the exact number of digits that are obtained. Thus they are never used to approximate π when speed or accuracy is desired. Spigot algorithms Two algorithms were discovered in 1995 that opened up new avenues of research into π. They are called spigot algorithms because, like water dripping from a spigot, they produce single digits of π that are not reused after they are calculated. This is in contrast to infinite series or iterative algorithms, which retain and use all intermediate digits until the final result is produced. American mathematicians Stan Wagon and Stanley Rabinowitz produced a simple spigot algorithm in 1995. Its speed is comparable to arctan algorithms, but not as fast as iterative algorithms. Another spigot algorithm, the BBP digit extraction algorithm, was discovered in 1995 by Simon Plouffe : π = ∑ k = 0 ∞ 1 16 k ( 4 8 k + 1 − 2 8 k + 4 − 1 8 k + 5 − 1 8 k + 6 ) ( displaystyle pi = sum _ ( k = 0 ) ^ ( infty ) ( \frac ( 1 ) ( 16 ^ ( k ) ) ) left ( ( \frac ( 4 ) ( 8k + 1 ) ) - ( \frac ( 2 ) ( 8k + 4 ) ) - ( \frac ( 1 ) ( 8k + 5 ) ) - ( \frac ( 1 ) ( 8k + 6 ) ) \right ) ) This formula, unlike others before it, can produce any individual hexadecimal digit of π without calculating all the preceding digits. Individual binary digits may be extracted from individual hexadecimal digits, and octal digits can be extracted from one or two hexadecimal digits. Variations of the algorithm have been discovered, but no digit extraction algorithm has yet been found that rapidly produces decimal digits. An important application of digit extraction algorithms is to validate new claims of record π computations : After a new record is claimed, the decimal result is converted to hexadecimal, and then a digit extraction algorithm is used to calculate several random hexadecimal digits near the end ; if they match, this provides a measure of confidence that the entire computation is correct. Between 1998 and 2000, the distributed computing project PiHex used Bellard 's formula ( a modification of the BBP algorithm ) to compute the quadrillionth ( 10 th ) bit of π, which turned out to be 0. In September 2010, a Yahoo ! employee used the company 's Hadoop application on one thousand computers over a 23 - day period to compute 256 bits of π at the two - quadrillionth ( 2 × 10 th ) bit, which also happens to be zero. Role and characterizations in mathematics Because π is closely related to the circle, it is found in many formulae from the fields of geometry and trigonometry, particularly those concerning circles, spheres, or ellipses. Other branches of science, such as statistics, physics, Fourier analysis, and number theory, also include π in some of their important formulae. Geometry and trigonometry The area of the circle equals π times the shaded area. π appears in formulae for areas and volumes of geometrical shapes based on circles, such as ellipses, spheres, cones, and tori. Below are some of the more common formulae that involve π. The circumference of a circle with radius r is 2πr. The area of a circle with radius r is πr. The volume of a sphere with radius r is 4 / 3πr. The surface area of a sphere with radius r is 4πr. The formulae above are special cases of the volume of the n - dimensional ball and the surface area of its boundary, the ( n − 1 ) - dimensional sphere, given below. Definite integrals that describe circumference, area, or volume of shapes generated by circles typically have values that involve π. For example, an integral that specifies half the area of a circle of radius one is given by : ∫ − 1 1 1 − x 2 d x = π 2. ( displaystyle int _ ( - 1 ) ^ ( 1 ) ( sqrt ( 1 - x ^ ( 2 ) ) ) , dx = ( \frac ( pi ) ( 2 ) ). ) In that integral the function √ 1 − x represents the top half of a circle ( the square root is a consequence of the Pythagorean theorem ), and the integral ∫ computes the area between that half of a circle and the x axis. Sine and cosine functions repeat with period 2π. The trigonometric functions rely on angles, and mathematicians generally use radians as units of measurement. π plays an important role in angles measured in radians, which are defined so that a complete circle spans an angle of 2π radians. The angle measure of 180 ° is equal to π radians, and 1 ° = π / 180 radians. Common trigonometric functions have periods that are multiples of π ; for example, sine and cosine have period 2π, so for any angle θ and any integer k, sin ⁡ θ = sin ⁡ ( θ + 2 π k ) and cos ⁡ θ = cos ⁡ ( θ + 2 π k ). ( displaystyle sin \theta = sin left ( \theta + 2 pi k \right ) ( \text ( and ) ) cos \theta = cos left ( \theta + 2 pi k \right ). ) Eigenvalues The overtones of a vibrating string are eigenfunctions of the second derivative, and form a harmonic progression. The associated eigenvalues form the arithmetic progression of integer multiples of π. Many of the appearances of π in the formulas of mathematics and the sciences have to do with its close relationship with geometry. However, π also appears in many natural situations having apparently nothing to do with geometry. In many applications, it plays a distinguished role as an eigenvalue. For example, an idealized vibrating string can be modelled as the graph of a function f on the unit interval ( 0, 1 ), with fixed ends f ( 0 ) = f ( 1 ) = 0. The modes of vibration of the string are solutions of the differential equation f `` ( x ) + λ f ( x ) = 0. Here λ is an associated eigenvalue, which is constrained by Sturm -- Liouville theory to take on only certain specific values. It must be positive, since the second derivative is negative definite, so it is convenient to write λ = ν where ν > 0 is called the wavenumber. Then f ( x ) = sin ( π x ) satisfies the boundary conditions and the differential equation with ν = π. The value π is, in fact, the least such value of the wavenumber, and is associated with the fundamental mode of vibration of the string. One way to obtain this is by estimating the energy. The energy satisfies an inequality, Wirtinger 's inequality for functions, which states that if a function f : ( 0, 1 ) → C is given such that f ( 0 ) = f ( 1 ) = 0 and f and f ' are both square integrable, then the inequality holds : π 2 ∫ 0 1 f ( x ) 2 d x ≤ ∫ 0 1 f ′ ( x ) 2 d x, ( displaystyle pi ^ ( 2 ) int _ ( 0 ) ^ ( 1 ) f ( x ) ^ ( 2 ) , dx leq int _ ( 0 ) ^ ( 1 ) f ' ( x ) ^ ( 2 ) , dx, ) and the case of equality holds precisely when f is a multiple of sin ( π x ). So π appears as an optimal constant in Wirtinger 's inequality, and from this it follows that it is the smallest such wavenumber, using the variational characterization of the eigenvalue. As a consequence, π is the smallest singular value of the derivative on the space of functions on ( 0, 1 ) vanishing at both endpoints ( the Sobolev space H 0 1 ( 0, 1 ) ( displaystyle H_ ( 0 ) ^ ( 1 ) ( 0, 1 ) ) ). Inequalities The ancient city of Carthage was the solution to an isoperimetric problem, according to a legend recounted by Lord Kelvin ( Thompson 1894 ) : those lands bordering the sea that Queen Dido could enclose on all other sides within a single given oxhide, cut into strips. The number π serves appears in similar eigenvalue problems in higher - dimensional analysis. As mentioned above, it can be characterized via its role as the best constant in the isoperimetric inequality : the area A enclosed by a plane Jordan curve of perimeter P satisfies the inequality 4 π A ≤ P 2, ( displaystyle 4 pi A leq P ^ ( 2 ), ) and equality is clearly achieved for the circle, since in that case A = πr and P = 2πr. Ultimately as a consequence of the isoperimetric inequality, π appears in the optimal constant for the critical Sobolev inequality in n dimensions, which thus characterizes the role of π in many physical phenomena as well, for example those of classical potential theory. In two dimensions, the critical Sobolev inequality is 2 π ‖ f ‖ 2 ≤ ‖ ∇ f ‖ 1 ( displaystyle 2 pi \f _ ( 2 ) leq \nabla f _ ( 1 ) ) for f a smooth function with compact support in R, ∇ f ( displaystyle \nabla f ) is the gradient of f, and ‖ f ‖ 2 ( displaystyle \f _ ( 2 ) ) and ‖ ∇ f ‖ 1 ( displaystyle \nabla f _ ( 1 ) ) refer respectively to the L and L - norm. The Sobolev inequality is equivalent to the isoperimetric inequality ( in any dimension ), with the same best constants. Wirtinger 's inequality also generalizes to higher - dimensional Poincaré inequalities that provide best constants for the Dirichlet energy of an n - dimensional membrane. Specifically, π is the greatest constant such that π ≤ ( ∫ G ∇ u 2 ) 1 / 2 ( ∫ G u 2 ) 1 / 2 ( displaystyle pi leq ( \frac ( left ( int _ ( G ) \nabla u ^ ( 2 ) \right ) ^ ( 1 / 2 ) ) ( left ( int _ ( G ) u ^ ( 2 ) \right ) ^ ( 1 / 2 ) ) ) ) for all convex subsets G of R of diameter 1, and square - integrable functions u on G of mean zero. Just as Wirtinger 's inequality is the variational form of the Dirichlet eigenvalue problem in one dimension, the Poincaré inequality is the variational form of the Neumann eigenvalue problem, in any dimension. Fourier transform and Heisenberg uncertainty principle An animation of a geodesic in the Heisenberg group, showing the close connection between the Heisenberg group, isoperimetry, and the constant π. The cumulative height of the geodesic is equal to the area of the shaded portion of the unit circle, while the arc length ( in the Carnot -- Carathéodory metric ) is equal to the circumference. The constant π also appears as a critical spectral parameter in the Fourier transform. This is the integral transform, that takes a complex - valued integrable function f on the real line to the function defined as : f ^ ( ξ ) = ∫ − ∞ ∞ f ( x ) e − 2 π i x ξ d x. ( displaystyle ( hat ( f ) ) ( \xi ) = int _ ( - infty ) ^ ( infty ) f ( x ) e ^ ( - 2 pi ix \xi ) , dx. ) There are several different conventions for the Fourier transform, all of which involve a factor of π that is placed somewhere. The appearance of π is essential in these formulas, as there is there is no possibility to remove π altogether from the Fourier transform and its inverse transform. The definition given above is the most canonical, however, because it describes the unique unitary operator on L that is also an algebra homomorphism of L to L. The Heisenberg uncertainty principle also contains the number π. The uncertainty principle gives a sharp lower bound on the extent to which it is possible to localize a function both in space and in frequency : with our conventions for the Fourier transform, ∫ − ∞ ∞ x 2 f ( x ) 2 d x ∫ − ∞ ∞ ξ 2 f ^ ( ξ ) 2 d ξ ≥ ( 1 4 π ∫ − ∞ ∞ f ( x ) 2 d x ) 2. ( displaystyle int _ ( - infty ) ^ ( infty ) x ^ ( 2 ) f ( x ) ^ ( 2 ) , dx int _ ( - infty ) ^ ( infty ) \xi ^ ( 2 ) ( hat ( f ) ) ( \xi ) ^ ( 2 ) , d \xi geq left ( ( \frac ( 1 ) ( 4 pi ) ) int _ ( - infty ) ^ ( infty ) f ( x ) ^ ( 2 ) , dx \right ) ^ ( 2 ). ) The physical consequence, about the uncertainty in simultaneous position and momentum observations of a quantum mechanical system, is discussed below. The appearance of π in the formulae of Fourier analysis is ultimately a consequence of the Stone -- von Neumann theorem, asserting the uniqueness of the Schrödinger representation of the Heisenberg group. Gaussian integrals A graph of the Gaussian function ƒ ( x ) = e. The colored region between the function and the x-axis has area √ π. The fields of probability and statistics frequently use the normal distribution as a simple model for complex phenomena ; for example, scientists generally assume that the observational error in most experiments follows a normal distribution. The Gaussian function, which is the probability density function of the normal distribution with mean μ and standard deviation σ, naturally contains π : f ( x ) = 1 σ 2 π e − ( x − μ ) 2 / ( 2 σ 2 ) ( displaystyle f ( x ) = ( 1 over sigma ( sqrt ( 2 pi ) ) ) , e ^ ( - ( x - mu ) ^ ( 2 ) / ( 2 sigma ^ ( 2 ) ) ) ) For this to be a probability density, the area under the graph of f needs to be equal to one. This follows from a change of variables in the Gaussian integral : ∫ − ∞ ∞ e − u 2 d u = π ( displaystyle int _ ( - infty ) ^ ( infty ) e ^ ( - u ^ ( 2 ) ) , du = ( sqrt ( pi ) ) ) which says that the area under the basic bell curve in the figure is equal to the square root of π. π can be computed from the distribution of zeros of a one - dimensional Wiener process The central limit theorem explains the central role of normal distributions, and thus of π, in probability and statistics. This theorem is ultimately connected with the spectral characterization of π as the eigenvalue associated with the Heisenberg uncertainty principle, and the fact that equality holds in the uncertainty principle only for the Gaussian function. Equivalently, π is the unique constant making the Gaussian normal distribution e equal to its own Fourier transform. Indeed, according to Howe ( 1980 ), the `` whole business '' of establishing the fundamental theorems of Fourier analysis reduces to the Gaussian integral. Projective geometry Let V be the set of all twice differentiable real functions f : R → R ( displaystyle f : mathbb ( R ) \to mathbb ( R ) ) that satisfy the ordinary differential equation f '' ( x ) + f ( x ) = 0 ( displaystyle f ' ' ( x ) + f ( x ) = 0 ). Then V is a two - dimensional real vector space, with two parameters corresponding to a pair of initial conditions for the differential equation. For any t ∈ R ( displaystyle t in mathbb ( R ) ), let e t : V → R ( displaystyle e_ ( t ) : V \to mathbb ( R ) ) be the evaluation functional, which associates to each f ∈ V ( displaystyle f in V ) the value e t ( f ) = f ( t ) ( displaystyle e_ ( t ) ( f ) = f ( t ) ) of the function f at the real point t. Then, for each t, the kernel of e t ( displaystyle e_ ( t ) ) is a one - dimensional linear subspace of V. Hence t ↦ ker ⁡ e t ( displaystyle t mapsto ker e_ ( t ) ) defines a function from R → P ( V ) ( displaystyle mathbb ( R ) \to mathbb ( P ) ( V ) ) from the real line to the real projective line. This function is periodic, and the quantity π can be characterized as the period of this map. Topology Uniformization of the Klein quartic, a surface of genus three and Euler characteristic − 4, as a quotient of the hyperbolic plane by the symmetry group PSL ( 2, 7 ) of the Fano plane. The hyperbolic area of a fundamental domain is 8π, by Gauss -- Bonnet. The constant π appears in the Gauss -- Bonnet formula which relates the differential geometry of surfaces to their topology. Specifically, if a compact surface Σ has Gauss curvature K, then ∫ Σ K d A = 2 π χ ( Σ ) ( displaystyle int _ ( Sigma ) K , dA = 2 pi chi ( Sigma ) ) where χ ( Σ ) is the Euler characteristic, which is an integer. An example is the surface area of a sphere S of curvature 1 ( so that its radius of curvature, which coincides with its radius, is also 1. ) The Euler characteristic of a sphere can be computed from its homology groups and is found to be equal to two. Thus we have A ( S ) = ∫ S 1 d A = 2 π ⋅ 2 = 4 π ( displaystyle A ( S ) = int _ ( S ) 1 , dA = 2 pi cdot 2 = 4 pi ) reproducing the formula for the surface area of a sphere of radius 1. The constant appears in many other integral formulae in topology, in particular, those involving characteristic classes via the Chern -- Weil homomorphism. Vector calculus The techniques of vector calculus can be understood in terms of decompositions into spherical harmonics ( shown ) Vector calculus is a branch of calculus that is concerned with the properties of vector fields, and has many physical applications such as to electricity and magnetism. The Newtonian potential for a point source Q situated at the origin of a three - dimensional Cartesian coordinate system is V ( x ) = − k Q x ( displaystyle V ( mathbf ( x ) ) = - ( \frac ( kQ ) ( mathbf ( x ) ) ) ) which represents the potential energy of a unit mass ( or charge ) placed a distance x from the source, and k is a dimensional constant. The field, denoted here by E, which may be the ( Newtonian ) gravitational field or the ( Coulomb ) electric field, is the negative gradient of the potential : E = − ∇ V. ( displaystyle mathbf ( E ) = - \nabla V. ) Special cases include Coulomb 's law and Newton 's law of universal gravitation. Gauss ' law states that the outward flux of the field through any smooth, simple, closed, orientable surface S containing the origin is equal to 4πkQ : 4 π k Q = ( displaystyle 4 pi kQ = ) S ( displaystyle ( scriptstyle S ) ) E ⋅ d A ( displaystyle mathbf ( E ) cdot d mathbf ( A ) ) It is standard to absorb this factor of 4π into the constant k, but this argument shows why it must appear somewhere. Furthermore, 4π is the surface area of the unit sphere, but we have not assumed that S is the sphere. However, as a consequence of the divergence theorem, because the region away from the origin is vacuum ( source - free ) it is only the homology class of the surface S in R ( 0 ) that matters in computing the integral, so it can be replaced by any convenient surface in the same homology class, in particular, a sphere, where spherical coordinates can be used to calculate the integral. A consequence of the Gauss law is that the negative Laplacian of the potential V is equal to 4πkQ times the Dirac delta function : Δ V ( x ) = − 4 π k Q δ ( x ). ( displaystyle Delta V ( mathbf ( x ) ) = - 4 pi kQ delta ( mathbf ( x ) ). ) More general distributions of matter ( or charge ) are obtained from this by convolution, giving the Poisson equation Δ V ( x ) = − 4 π k ρ ( x ) ( displaystyle Delta V ( mathbf ( x ) ) = - 4 pi k \rho ( mathbf ( x ) ) ) where ρ is the distribution function. Einstein 's equation states that the curvature of space - time is produced by the matter - energy content. The constant π also plays an analogous role in four - dimensional potentials associated with Einstein 's equations, a fundamental formula which forms the basis of the general theory of relativity and describes the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy : R μ ν − 1 2 R g μ ν + Λ g μ ν = 8 π G c 4 T μ ν, ( displaystyle R_ ( mu \nu ) - ( \frac ( 1 ) ( 2 ) ) Rg_ ( mu \nu ) + Lambda g_ ( mu \nu ) = ( \frac ( 8 pi G ) ( c ^ ( 4 ) ) ) T_ ( mu \nu ), ) where R is the Ricci curvature tensor, R is the scalar curvature, g is the metric tensor, Λ is the cosmological constant, G is Newton 's gravitational constant, c is the speed of light in vacuum, and T is the stress -- energy tensor. The left - hand side of Einstein 's equation is a non-linear analog of the Laplacian of the metric tensor, and reduces to that in the weak field limit, with the Λ g ( displaystyle Lambda g ) term playing the role of a Lagrange multiplier, and the right - hand side is the analog of the distribution function, times 8π. Cauchy 's integral formula Complex analytic functions can be visualized as a collection of streamlines and equipotentials, systems of curves intersecting at right angles. Here illustrated is the complex logarithm of the Gamma function. One of the key tools in complex analysis is contour integration of a function over a positively oriented ( rectifiable ) Jordan curve γ. A form of Cauchy 's integral formula states that if a point z is interior to γ, then ∮ γ ⁡ d z z − z 0 = 2 π i. ( displaystyle oint _ ( gamma ) ( \frac ( dz ) ( z - z_ ( 0 ) ) ) = 2 pi i. ) Although the curve γ is not a circle, and hence does not have any obvious connection to the constant π, a standard proof of this result uses Morera 's theorem, which implies that the integral is invariant under homotopy of the curve, so that it can be deformed to a circle and then integrated explicitly in polar coordinates. More generally, it is true that if a rectifiable closed curve γ does not contain z, then the above integral is 2πi times the winding number of the curve. The general form of Cauchy 's integral formula establishes the relationship between the values of a complex analytic function f ( z ) on the Jordan curve γ and the value of f ( z ) at any interior point z of γ : ∮ γ ⁡ f ( z ) z − z 0 d z = 2 π i f ( z 0 ) ( displaystyle oint _ ( gamma ) ( f ( z ) over z - z_ ( 0 ) ) , dz = 2 pi if ( z_ ( 0 ) ) ) provided f ( z ) is analytic in the region enclosed by γ and extends continuously to γ. Cauchy 's integral formula is a special case of the residue theorem, that if g ( z ) is a meromorphic function the region enclosed by γ and is continuous in a neighborhood of γ, then ∮ γ ⁡ g ( z ) d z = 2 π i ∑ Res ⁡ ( g, a k ) ( displaystyle oint _ ( gamma ) g ( z ) , dz = 2 pi i sum operatorname ( Res ) ( g, a_ ( k ) ) ) where the sum is of the residues at the poles of g ( z ). The gamma function and Stirling 's approximation The Hopf fibration of the 3 - sphere, by Villarceau circles, over the complex projective line with its Fubini -- Study metric ( three parallels are shown ). The identity S ( 1 ) / S ( 1 ) = π / 2 is a consequence. The factorial function n ! is the product of all of the positive integers through n. The gamma function extends the concept of factorial ( normally defined only for non-negative integers ) to all complex numbers, except the negative real integers. When the gamma function is evaluated at half - integers, the result contains π ; for example Γ ( 1 / 2 ) = π ( displaystyle Gamma ( 1 / 2 ) = ( sqrt ( pi ) ) ) and Γ ( 5 / 2 ) = 3 π 4 ( displaystyle Gamma ( 5 / 2 ) = ( \frac ( 3 ( sqrt ( pi ) ) ) ( 4 ) ) ). The gamma function is defined by its Weierstrass product development : Γ ( z ) = e − γ z ∏ n = 1 ∞ e z / n 1 + z / n ( displaystyle Gamma ( z ) = e ^ ( - gamma z ) prod _ ( n = 1 ) ^ ( infty ) ( \frac ( e ^ ( z / n ) ) ( 1 + z / n ) ) ) where γ is the Euler -- Mascheroni constant. Evaluated at z = 1 / 2 and squared, the equation Γ ( 1 / 2 ) = π reduces to the Wallis product formula. The gamma function is also connected to the Riemann zeta function and identities for the functional determinant, in which the constant π plays an important role. The gamma function is used to calculate the volume V ( r ) of the n - dimensional ball of radius r in Euclidean n - dimensional space, and the surface area S ( r ) of its boundary, the ( n − 1 ) - dimensional sphere : V n ( r ) = π n / 2 Γ ( n 2 + 1 ) r n ( displaystyle V_ ( n ) ( r ) = ( \frac ( pi ^ ( n / 2 ) ) ( Gamma ( ( \frac ( n ) ( 2 ) ) + 1 ) ) ) r ^ ( n ) ) S n − 1 ( r ) = n π n / 2 Γ ( n 2 + 1 ) r n − 1 ( displaystyle S_ ( n - 1 ) ( r ) = ( \frac ( n pi ^ ( n / 2 ) ) ( Gamma ( ( \frac ( n ) ( 2 ) ) + 1 ) ) ) r ^ ( n - 1 ) ) Further, it follows from the functional equation that 2 π r = S n + 1 ( r ) V n ( r ). ( displaystyle 2 pi r = ( \frac ( S_ ( n + 1 ) ( r ) ) ( V_ ( n ) ( r ) ) ). ) The gamma function can be used to create a simple approximation to the factorial function n ! for large n : n ! ∼ 2 π n ( n e ) n ( displaystyle n ! sim ( sqrt ( 2 pi n ) ) left ( ( \frac ( n ) ( e ) ) \right ) ^ ( n ) ) which is known as Stirling 's approximation. Equivalently, π = lim n → ∞ e 2 n n ! 2 2 n 2 n + 1. ( displaystyle pi = lim _ ( n \to infty ) ( \frac ( e ^ ( 2n ) n ! ^ ( 2 ) ) ( 2n ^ ( 2n + 1 ) ) ). ) As a geometrical application of Stirling 's approximation, let Δ denote the standard simplex in n - dimensional Euclidean space, and ( n + 1 ) Δ denote the simplex having all of its sides scaled up by a factor of n + 1. Then Vol ⁡ ( ( n + 1 ) Δ n ) = ( n + 1 ) n n ! ∼ e n + 1 2 π n. ( displaystyle operatorname ( Vol ) ( ( n + 1 ) Delta _ ( n ) ) = ( \frac ( ( n + 1 ) ^ ( n ) ) ( n ! ) ) sim ( \frac ( e ^ ( n + 1 ) ) ( sqrt ( 2 pi n ) ) ). ) Ehrhart 's volume conjecture is that this is the ( optimal ) upper bound on the volume of a convex body containing only one lattice point. Number theory and Riemann zeta function Each prime has an associated Prüfer group, which are arithmetic localizations of the circle. The L - functions of analytic number theory are also localized in each prime p. Solution of the Basel problem using the Weil conjecture : the value of ζ ( 2 ) ( displaystyle zeta ( 2 ) ) is the hyperbolic area of a fundamental domain of the modular group, times 2 π ( displaystyle 2 pi ) The Riemann zeta function ζ ( s ) is used in many areas of mathematics. When evaluated at s = 2 it can be written as ζ ( 2 ) = 1 1 2 + 1 2 2 + 1 3 2 + ⋯ ( displaystyle zeta ( 2 ) = ( \frac ( 1 ) ( 1 ^ ( 2 ) ) ) + ( \frac ( 1 ) ( 2 ^ ( 2 ) ) ) + ( \frac ( 1 ) ( 3 ^ ( 2 ) ) ) + cdots ) Finding a simple solution for this infinite series was a famous problem in mathematics called the Basel problem. Leonhard Euler solved it in 1735 when he showed it was equal to π / 6. Euler 's result leads to the number theory result that the probability of two random numbers being relatively prime ( that is, having no shared factors ) is equal to 6 / π. This probability is based on the observation that the probability that any number is divisible by a prime p is 1 / p ( for example, every 7th integer is divisible by 7. ) Hence the probability that two numbers are both divisible by this prime is 1 / p, and the probability that at least one of them is not is 1 − 1 / p. For distinct primes, these divisibility events are mutually independent ; so the probability that two numbers are relatively prime is given by a product over all primes : ∏ p ∞ ( 1 − 1 p 2 ) = ( ∏ p ∞ 1 1 − p − 2 ) − 1 = 1 1 + 1 2 2 + 1 3 2 + ⋯ = 1 ζ ( 2 ) = 6 π 2 ≈ 61 %. ( displaystyle ( \begin ( aligned ) prod _ ( p ) ^ ( infty ) left ( 1 - ( \frac ( 1 ) ( p ^ ( 2 ) ) ) \right ) & = left ( prod _ ( p ) ^ ( infty ) ( \frac ( 1 ) ( 1 - p ^ ( - 2 ) ) ) \right ) ^ ( - 1 ) ( 4pt ) & = ( \frac ( 1 ) ( 1 + ( \frac ( 1 ) ( 2 ^ ( 2 ) ) ) + ( \frac ( 1 ) ( 3 ^ ( 2 ) ) ) + cdots ) ) ( 4pt ) & = ( \frac ( 1 ) ( zeta ( 2 ) ) ) = ( \frac ( 6 ) ( pi ^ ( 2 ) ) ) approx 61 %. end ( aligned ) ) ) This probability can be used in conjunction with a random number generator to approximate π using a Monte Carlo approach. The solution to the Basel problem implies that the geometrically derived quantity π is connected in a deep way to the distribution of prime numbers. This is a special case of Weil 's conjecture on Tamagawa numbers, which asserts the equality of similar such infinite products of arithmetic quantities, localized at each prime p, and a geometrical quantity : the reciprocal of the volume of a certain locally symmetric space. In the case of the Basel problem, it is the hyperbolic 3 - manifold SL ( R ) / SL ( Z ). The zeta function also satisfies Riemann 's functional equation, which involves π as well as the gamma function : ζ ( s ) = 2 s π s − 1 sin ⁡ ( π s 2 ) Γ ( 1 − s ) ζ ( 1 − s ). ( displaystyle zeta ( s ) = 2 ^ ( s ) pi ^ ( s - 1 ) sin left ( ( \frac ( pi s ) ( 2 ) ) \right ) Gamma ( 1 - s ) zeta ( 1 - s ) !. ) Furthermore, the derivative of the zeta function satisfies exp ⁡ ( − ζ ′ ( 0 ) ) = 2 π. ( displaystyle exp ( - zeta ' ( 0 ) ) = ( sqrt ( 2 pi ) ). ) A consequence is that π can be obtained from the functional determinant of the harmonic oscillator. This functional determinant can be computed via a product expansion, and is equivalent to the Wallis product formula. The calculation can be recast in quantum mechanics, specifically the variational approach to the spectrum of the hydrogen atom. Fourier series π appears in characters of p - adic numbers ( shown ), which are elements of a Prüfer group. Tate 's thesis makes heavy use of this machinery. The constant π also appears naturally in Fourier series of periodic functions. Periodic functions are functions on the group T = R / Z of fractional parts of real numbers. The Fourier decomposition shows that a complex - valued function f on T can be written as an infinite linear superposition of unitary characters of T. That is, continuous group homomorphisms from T to the circle group U ( 1 ) of unit modulus complex numbers. It is a theorem that every character of T is one of the complex exponentials e n ( x ) = e 2 π i n x ( displaystyle e_ ( n ) ( x ) = e ^ ( 2 pi inx ) ). There is a unique character on T, up to complex conjugation, that is a group isomorphism. Using the Haar measure on the circle group, the constant π is half the magnitude of the Radon -- Nikodym derivative of this character. The other characters have derivatives whose magnitudes are positive integral multiples of 2π. As a result, the constant π is the unique number such that the group T, equipped with its Haar measure, is Pontrjagin dual to the lattice of integral multiples of 2π. This is a version of the one - dimensional Poisson summation formula. Modular forms and theta functions Theta functions transform under the lattice of periods of an elliptic curve. The constant π is connected in a deep way with the theory of modular forms and theta functions. For example, the Chudnovsky algorithm involves in an essential way the j - invariant of an elliptic curve. Modular forms are holomorphic functions in the upper half plane characterized by their transformation properties under the modular group S L 2 ( Z ) ( displaystyle mathrm ( SL ) _ ( 2 ) ( mathbb ( Z ) ) ) ( or its various subgroups ), a lattice in the group S L 2 ( R ) ( displaystyle mathrm ( SL ) _ ( 2 ) ( mathbb ( R ) ) ). An example is the Jacobi theta function θ ( z, τ ) = ∑ n = − ∞ ∞ e 2 π i n z + i π n 2 τ ( displaystyle \theta ( z, \tau ) = sum _ ( n = - infty ) ^ ( infty ) e ^ ( 2 pi inz + i pi n ^ ( 2 ) \tau ) ) which is a kind of modular form called a Jacobi form. This is sometimes written in terms of the nome q = e π i τ ( displaystyle q = e ^ ( pi i \tau ) ). The constant π is the unique constant making the Jacobi theta function an automorphic form, which means that it transforms in a specific way. Certain identities hold for all automorphic forms. An example is θ ( z + τ, τ ) = e − π i τ − 2 π i z θ ( z, τ ), ( displaystyle \theta ( z+ \tau, \tau ) = e ^ ( - pi i \tau - 2 pi iz ) \theta ( z, \tau ), ) which implies that θ transforms as a representation under the discrete Heisenberg group. General modular forms and other theta functions also involve π, once again because of the Stone -- von Neumann theorem. Cauchy distribution and potential theory The Witch of Agnesi, named for Maria Agnesi ( 1718 -- 1799 ), is a geometrical construction of the graph of the Cauchy distribution. The Cauchy distribution g ( x ) = 1 π ⋅ 1 x 2 + 1 ( displaystyle g ( x ) = ( \frac ( 1 ) ( pi ) ) cdot ( \frac ( 1 ) ( x ^ ( 2 ) + 1 ) ) ) is a probability density function. The total probability is equal to one, owing to the integral : ∫ − ∞ ∞ 1 x 2 + 1 d x = π. ( displaystyle int _ ( - infty ) ^ ( infty ) ( \frac ( 1 ) ( x ^ ( 2 ) + 1 ) ) , dx = pi. ) The Shannon entropy of the Cauchy distribution is equal to log ( 4π ), which also involves π. The Cauchy distribution governs the passage of Brownian particles through a membrane. The Cauchy distribution plays an important role in potential theory because it is the simplest Furstenberg measure, the classical Poisson kernel associated with a Brownian motion in a half - plane. Conjugate harmonic functions and so also the Hilbert transform are associated with the asymptotics of the Poisson kernel. The Hilbert transform H is the integral transform given by the Cauchy principal value of the singular integral H f ( t ) = 1 π ∫ − ∞ ∞ f ( x ) d x x − t. ( displaystyle Hf ( t ) = ( \frac ( 1 ) ( pi ) ) int _ ( - infty ) ^ ( infty ) ( \frac ( f ( x ) , dx ) ( x-t ) ). ) The constant π is the unique ( positive ) normalizing factor such that H defines a linear complex structure on the Hilbert space of square - integrable real - valued functions on the real line. The Hilbert transform, like the Fourier transform, can be characterized purely in terms of its transformation properties on the Hilbert space L ( R ) : up to a normalization factor, it is the unique bounded linear operator that commutes with positive dilations and anti-commutes with all reflections of the real line. The constant π is the unique normalizing factor that makes this transformation unitary. Complex dynamics π can be computed from the Mandelbrot set, by counting the number of iterations required before point ( − 0.75, ε ) diverges. An occurrence of π in the Mandelbrot set fractal was discovered by David Boll in 1991. He examined the behavior of the Mandelbrot set near the `` neck '' at ( − 0.75, 0 ). If points with coordinates ( − 0.75, ε ) are considered, as ε tends to zero, the number of iterations until divergence for the point multiplied by ε converges to π. The point ( 0.25, ε ) at the cusp of the large `` valley '' on the right side of the Mandelbrot set behaves similarly : the number of iterations until divergence multiplied by the square root of ε tends to π. Outside mathematics Describing physical phenomena Although not a physical constant, π appears routinely in equations describing fundamental principles of the universe, often because of π 's relationship to the circle and to spherical coordinate systems. A simple formula from the field of classical mechanics gives the approximate period T of a simple pendulum of length L, swinging with a small amplitude ( g is the earth 's gravitational acceleration ) : T ≈ 2 π L g. ( displaystyle T approx 2 pi ( sqrt ( \frac ( L ) ( g ) ) ). ) One of the key formulae of quantum mechanics is Heisenberg 's uncertainty principle, which shows that the uncertainty in the measurement of a particle 's position ( Δx ) and momentum ( Δp ) can not both be arbitrarily small at the same time ( where h is Planck 's constant ) : Δ x Δ p ≥ h 4 π. ( displaystyle Delta x , Delta p geq ( \frac ( h ) ( 4 pi ) ). ) The fact that π is approximately equal to 3 plays a role in the relatively long lifetime of orthopositronium. The inverse lifetime to lowest order in the fine - structure constant α is 1 τ = 2 π 2 − 9 9 π m α 6, ( displaystyle ( \frac ( 1 ) ( \tau ) ) = 2 ( \frac ( pi ^ ( 2 ) - 9 ) ( 9 pi ) ) m alpha ^ ( 6 ), ) where m is the mass of the electron. π is present in some structural engineering formulae, such as the buckling formula derived by Euler, which gives the maximum axial load F that a long, slender column of length L, modulus of elasticity E, and area moment of inertia I can carry without buckling : F = π 2 E I L 2. ( displaystyle F = ( \frac ( pi ^ ( 2 ) EI ) ( L ^ ( 2 ) ) ). ) The field of fluid dynamics contains π in Stokes ' law, which approximates the frictional force F exerted on small, spherical objects of radius R, moving with velocity v in a fluid with dynamic viscosity η : F = 6 π η R v. ( displaystyle F = 6 , pi , eta , R , v. ) Under ideal conditions ( uniform gentle slope on a homogeneously erodible substrate ), the sinuosity of a meandering river approaches π. The sinuosity is the ratio between the actual length and the straight - line distance from source to mouth. Faster currents along the outside edges of a river 's bends cause more erosion than along the inside edges, thus pushing the bends even farther out, and increasing the overall loopiness of the river. However, that loopiness eventually causes the river to double back on itself in places and `` short - circuit '', creating an ox - bow lake in the process. The balance between these two opposing factors leads to an average ratio of π between the actual length and the direct distance between source and mouth. Memorizing digits Main article : Piphilology Piphilology is the practice of memorizing large numbers of digits of π, and world - records are kept by the Guinness World Records. The record for memorizing digits of π, certified by Guinness World Records, is 70,000 digits, recited in India by Rajveer Meena in 9 hours and 27 minutes on 21 March 2015. In 2006, Akira Haraguchi, a retired Japanese engineer, claimed to have recited 100,000 decimal places, but the claim was not verified by Guinness World Records. One common technique is to memorize a story or poem in which the word lengths represent the digits of π : The first word has three letters, the second word has one, the third has four, the fourth has one, the fifth has five, and so on. An early example of a memorization aid, originally devised by English scientist James Jeans, is `` How I want a drink, alcoholic of course, after the heavy lectures involving quantum mechanics. '' When a poem is used, it is sometimes referred to as a piem. Poems for memorizing π have been composed in several languages in addition to English. Record - setting π memorizers typically do not rely on poems, but instead use methods such as remembering number patterns and the method of loci. A few authors have used the digits of π to establish a new form of constrained writing, where the word lengths are required to represent the digits of π. The Cadaeic Cadenza contains the first 3835 digits of π in this manner, and the full - length book Not a Wake contains 10,000 words, each representing one digit of π. In popular culture A pi pie. The circular shape of pie makes it a frequent subject of pi puns. Perhaps because of the simplicity of its definition and its ubiquitous presence in formulae, π has been represented in popular culture more than other mathematical constructs. In the 2008 Open University and BBC documentary co-production, The Story of Maths, aired in October 2008 on BBC Four, British mathematician Marcus du Sautoy shows a visualization of the -- historically first exact -- formula for calculating π when visiting India and exploring its contributions to trigonometry. In the Palais de la Découverte ( a science museum in Paris ) there is a circular room known as the pi room. On its wall are inscribed 707 digits of π. The digits are large wooden characters attached to the dome - like ceiling. The digits were based on an 1853 calculation by English mathematician William Shanks, which included an error beginning at the 528th digit. The error was detected in 1946 and corrected in 1949. In Carl Sagan 's novel Contact it is suggested that the creator of the universe buried a message deep within the digits of π. The digits of π have also been incorporated into the lyrics of the song `` Pi '' from the album Aerial by Kate Bush. In the United States, Pi Day falls on 14 March ( written 3 / 14 in the US style ), and is popular among students. π and its digital representation are often used by self - described `` math geeks '' for inside jokes among mathematically and technologically minded groups. Several college cheers at the Massachusetts Institute of Technology include `` 3.14159 ''. Pi Day in 2015 was particularly significant because the date and time 3 / 14 / 15 9 : 26 : 53 reflected many more digits of pi. During the 2011 auction for Nortel 's portfolio of valuable technology patents, Google made a series of unusually specific bids based on mathematical and scientific constants, including π. In 1958 Albert Eagle proposed replacing π by τ ( tau ), where τ = π / 2, to simplify formulas. However, no other authors are known to use τ in this way. Some people use a different value, τ = 6.283185... = 2π, arguing that τ, as the number of radians in one turn or as the ratio of a circle 's circumference to its radius rather than its diameter, is more natural than π and simplifies many formulas. Celebrations of this number, because it approximately equals 6.28, by making 28 June `` Tau Day '' and eating `` twice the pie '', have been reported in the media. However, this use of τ has not made its way into mainstream mathematics. In 1897, an amateur American mathematician attempted to persuade the Indiana legislature to pass the Indiana Pi Bill, which described a method to square the circle and contained text that implied various incorrect values for π, including 3.2. The bill is notorious as an attempt to establish a value of scientific constant by legislative fiat. The bill was passed by the Indiana House of Representatives, but rejected by the Senate, meaning it did not become a law. In computer culture In contemporary internet culture, individuals and organizations frequently pay homage to the number π. For instance, the computer scientist Donald Knuth let the version numbers of his program TeX approach π. The versions are 3, 3.1, 3.14, and so forth. Notes Footnotes Jump up ^ http://mathworld.wolfram.com/Pi.html Jump up ^ Andrews, George E. ; Askey, Richard ; Roy, Ranjan ( 1999 ). Special Functions. Cambridge University Press. p. 58. ISBN 0 - 521 - 78988 - 5. Jump up ^ Gupta, R.C. ( 1992 ). `` On the remainder term in the Madhava -- Leibniz 's series ''. Ganita Bharati. 14 ( 1 -- 4 ) : 68 -- 71. Jump up ^ π trillion digits of π Archived 6 December 2016 at the Wayback Machine. -- the exact number of digits increases periodically -- it should not be included in this article by citing only a primary reference source. Jump up ^ Arndt & Haenel 2006, p. 17 Jump up ^ Bailey, David ; Borwein, Jonathan ; Borwein, Peter ; Plouffe, Simon ( 1997 ). `` The Quest for Pi ''. The Mathematical Intelligencer. 19 ( 1 ) : 50 -- 56. CiteSeerX 10.1. 1.138. 7085. doi : 10.1007 / bf03024340. Jump up ^ Boeing, Niels ( 14 March 2016 ). `` Die Welt ist Pi '' ( The World is Pi ). Zeit Online ( in German ). Archived from the original on 17 March 2016. Die Ludolphsche Zahl oder Kreiszahl erhielt nun auch das Symbol, unter dem wir es heute kennen : William Jones schlug 1706 den griechischen Buchstaben π vor, in Anlehnung an perimetros, griechisch für Umfang. Leonhard Euler etablierte π schließlich in seinen mathematischen Schriften. ( The Ludolphian number or circle number now also received the symbol under which we know it today : William Jones proposed in 1706 the Greek letter π, based on perimetros ( περίμετρος ), Greek for perimeter. Leonhard Euler firmly established π in his mathematical writings. ) Jump up ^ `` pi ''. Dictionary.reference.com. 2 March 1993. Archived from the original on 28 July 2014. Retrieved 18 June 2012. ^ Jump up to : Arndt & Haenel 2006, p. 8 Jump up ^ Apostol, Tom ( 1967 ). Calculus, volume 1 ( 2nd ed. ). Wiley... p. 102 : `` From a logical point of view, this is unsatisfactory at the present stage because we have not yet discussed the concept of arc length. '' Arc length is introduced on p. 529. ^ Jump up to : Remmert, Reinhold ( 1991 ), `` What is π ? '', Numbers, Springer, p. 129 Jump up ^ Remmert ( 1991 ). The precise integral that Weierstrass used was π = ∫ − ∞ ∞ d x 1 + x 2. ( displaystyle pi = int _ ( - infty ) ^ ( infty ) ( \frac ( dx ) ( 1 + x ^ ( 2 ) ) ). ) Jump up ^ Baltzer, Richard ( 1870 ), Die Elemente der Mathematik ( The Elements of Mathematics ) ( in German ), Hirzel, p. 195, archived from the original on 14 September 2016 Jump up ^ Landau, Edmund ( 1934 ), Einführung in die Differentialrechnung und Integralrechnung ( in German ), Noordoff, p. 193 ^ Jump up to : Rudin, Walter ( 1976 ). Principles of Mathematical Analysis. McGraw - Hill. ISBN 0 - 07 - 054235 - X., p. 183. Jump up ^ Rudin, Walter ( 1986 ). Real and complex analysis. McGraw - Hill., p. 2. Jump up ^ Ahlfors, Lars ( 1966 ), Complex analysis, McGraw - Hill, p. 46 Jump up ^ Bourbaki, Nicolas ( 1981 ), Topologie generale, Springer, § VIII. 2. ^ Jump up to : Bourbaki, Nicolas ( 1979 ), Fonctions d'une variable réelle ( in French ), Springer, § II. 3. ^ Jump up to : Arndt & Haenel 2006, p. 5 Jump up ^ Salikhov, V. ( 2008 ). `` On the Irrationality Measure of pi ''. Russian Mathematical Survey. 53 ( 3 ) : 570 -- 572. Bibcode : 2008RuMaS... 63... 570S. doi : 10.1070 / RM2008v063n03ABEH004543. ^ Jump up to : Arndt & Haenel 2006, pp. 22 -- 23 Preuss, Paul ( 23 July 2001 ). `` Are The Digits of Pi Random ? Lab Researcher May Hold The Key ''. Lawrence Berkeley National Laboratory. Archived from the original on 20 October 2007. Retrieved 10 November 2007. Jump up ^ Arndt & Haenel 2006, pp. 22, 28 -- 30 Jump up ^ Arndt & Haenel 2006, p. 3 Jump up ^ Mayer, Steve. `` The Transcendence of π ''. Archived from the original on 2000 - 09 - 29. Retrieved 4 November 2007. Jump up ^ The polynomial shown is the first few terms of the Taylor series expansion of the sine function. Jump up ^ Posamentier & Lehmann 2004, p. 25 Jump up ^ Eymard & Lafon 1999, p. 129 Jump up ^ Beckmann 1989, p. 37 Schlager, Neil ; Lauer, Josh ( 2001 ). Science and Its Times : Understanding the Social Significance of Scientific Discovery. Gale Group. ISBN 0 - 7876 - 3933 - 8., p. 185. ^ Jump up to : Eymard & Lafon 1999, p. 78 Jump up ^ Sloane, N.J.A. ( ed. ). `` Sequence A001203 ( Continued fraction for Pi ) ''. The On - Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 12 April 2012. Jump up ^ Lange, L.J. ( May 1999 ). `` An Elegant Continued Fraction for π ''. The American Mathematical Monthly. 106 ( 5 ) : 456 -- 458. doi : 10.2307 / 2589152. JSTOR 2589152. Jump up ^ Arndt & Haenel 2006, p. 240 Jump up ^ Arndt & Haenel 2006, p. 242 Jump up ^ Kennedy, E.S., `` Abu - r - Raihan al - Biruni, 973 -- 1048 '', Journal for the History of Astronomy, 9 : 65, Bibcode : 1978JHA... 9... 65K, doi : 10.1177 / 002182867800900106. Ptolemy used a three - sexagesimal - digit approximation, and Jamshīd al - Kāshī expanded this to nine digits ; see Aaboe, Asger ( 1964 ), Episodes from the Early History of Mathematics, New Mathematical Library, 13, New York : Random House, p. 125, archived from the original on 29 November 2016 Jump up ^ Ayers 1964, p. 100 ^ Jump up to : Bronshteĭn & Semendiaev 1971, p. 592 Jump up ^ Maor, Eli, E : The Story of a Number, Princeton University Press, 2009, p. 160, ISBN 978 - 0 - 691 - 14134 - 3 ( `` five most important '' constants ). Jump up ^ Weisstein, Eric W. `` Roots of Unity ''. MathWorld. Jump up ^ Petrie, W.M.F. Wisdom of the Egyptians ( 1940 ) Jump up ^ Based on the Great Pyramid of Giza, supposedly built so that the circle whose radius is equal to the height of the pyramid has a circumference equal to the perimeter of the base ( it is 1760 cubits around and 280 cubits in height ). Verner, Miroslav. The Pyramids : The Mystery, Culture, and Science of Egypt 's Great Monuments. Grove Press. 2001 ( 1997 ). ISBN 0 - 8021 - 3935 - 3 Jump up ^ Rossi, Corinna Architecture and Mathematics in Ancient Egypt, Cambridge University Press. 2007. ISBN 978 - 0 - 521 - 69053 - 9. Jump up ^ Legon, J.A.R. On Pyramid Dimensions and Proportions ( 1991 ) Discussions in Egyptology ( 20 ) 25 -- 34 `` Archived copy ''. Archived from the original on 18 July 2011. Retrieved 7 June 2011. Jump up ^ `` We can conclude that although the ancient Egyptians could not precisely define the value of π, in practice they used it ''. Verner, M. ( 2003 ). `` The Pyramids : Their Archaeology and History ''., p. 70. Petrie ( 1940 ). `` Wisdom of the Egyptians ''., p. 30. See also Legon, J.A.R. ( 1991 ). `` On Pyramid Dimensions and Proportions ''. Discussions in Egyptology. 20 : 25 -- 34. Archived from the original on 18 July 2011... See also Petrie, W.M.F. ( 1925 ). `` Surveys of the Great Pyramids ''. Nature. 116 ( 2930 ) : 942. Bibcode : 1925Natur. 116... 942P. doi : 10.1038 / 116942a0. Jump up ^ Egyptologist : Rossi, Corinna, Architecture and Mathematics in Ancient Egypt, Cambridge University Press, 2004, pp. 60 -- 70, 200, ISBN 9780521829540. Skeptics : Shermer, Michael, The Skeptic Encyclopedia of Pseudoscience, ABC - CLIO, 2002, pp. 407 -- 408, ISBN 9781576076538. See also Fagan, Garrett G., Archaeological Fantasies : How Pseudoarchaeology Misrepresents The Past and Misleads the Public, Routledge, 2006, ISBN 9780415305938. For a list of explanations for the shape that do not involve π, see Herz - Fischler, Roger ( 2000 ). The Shape of the Great Pyramid. Wilfrid Laurier University Press. pp. 67 -- 77, 165 -- 166. ISBN 9780889203242. Archived from the original on 29 November 2016. Retrieved 5 June 2013. ^ Jump up to : Arndt & Haenel 2006, p. 167 Jump up ^ Chaitanya, Krishna. A profile of Indian culture. Indian Book Company ( 1975 ). p. 133. Jump up ^ Arndt & Haenel 2006, p. 169 Jump up ^ Arndt & Haenel 2006, p. 170 Jump up ^ Arndt & Haenel 2006, pp. 175, 205 Jump up ^ `` The Computation of Pi by Archimedes : The Computation of Pi by Archimedes -- File Exchange -- MATLAB Central ''. Mathworks.com. Archived from the original on 25 February 2013. Retrieved 12 March 2013. Jump up ^ Arndt & Haenel 2006, p. 171 Jump up ^ Arndt & Haenel 2006, p. 176 Boyer & Merzbach 1991, p. 168 Jump up ^ Arndt & Haenel 2006, pp. 15 -- 16, 175, 184 -- 186, 205. Grienberger achieved 39 digits in 1630 ; Sharp 71 digits in 1699. Jump up ^ Arndt & Haenel 2006, pp. 176 -- 177 ^ Jump up to : Boyer & Merzbach 1991, p. 202 Jump up ^ Arndt & Haenel 2006, p. 177 Jump up ^ Arndt & Haenel 2006, p. 178 Jump up ^ Arndt & Haenel 2006, pp. 179 ^ Jump up to : Arndt & Haenel 2006, pp. 180 Jump up ^ Azarian, Mohammad K. ( 2010 ). `` al - Risāla al - muhītīyya : A Summary ''. Missouri Journal of Mathematical Sciences. 22 ( 2 ) : 64 -- 85. Archived from the original on 14 January 2015. Jump up ^ O'Connor, John J. ; Robertson, Edmund F. ( 1999 ). `` Ghiyath al - Din Jamshid Mas'ud al - Kashi ''. MacTutor History of Mathematics archive. Archived from the original on 12 April 2011. Retrieved 11 August 2012. ^ Jump up to : Arndt & Haenel 2006, p. 182 Jump up ^ Arndt & Haenel 2006, pp. 182 -- 183 ^ Jump up to : Arndt & Haenel 2006, p. 183 Jump up ^ Grienbergerus, Christophorus ( 1630 ). Elementa Trigonometrica ( PDF ) ( in Latin ). Archived from the original ( PDF ) on 2014 - 02 - 01. His evaluation was 3.14159 26535 89793 23846 26433 83279 50288 4196 < π < 3.14159 26535 89793 23846 26433 83279 50288 4199. ^ Jump up to : Arndt & Haenel 2006, pp. 185 -- 191 Jump up ^ Roy 1990, pp. 101 -- 102 Arndt & Haenel 2006, pp. 185 -- 186 ^ Jump up to : Roy 1990, pp. 101 -- 102 Jump up ^ Joseph 1991, p. 264 ^ Jump up to : Arndt & Haenel 2006, p. 188. Newton quoted by Arndt. ^ Jump up to : Arndt & Haenel 2006, p. 187 Jump up ^ A060294 Jump up ^ Variorum de rebus mathematicis responsorum liber VIII. Jump up ^ Arndt & Haenel 2006, pp. 188 -- 189 ^ Jump up to : Eymard & Lafon 1999, pp. 53 -- 54 Jump up ^ Arndt & Haenel 2006, p. 189 Jump up ^ Arndt & Haenel 2006, p. 156 Jump up ^ Arndt & Haenel 2006, pp. 192 -- 193 ^ Jump up to : Arndt & Haenel 2006, pp. 72 -- 74 Jump up ^ Arndt & Haenel 2006, pp. 192 -- 196, 205 ^ Jump up to : Arndt & Haenel 2006, pp. 194 -- 196 ^ Jump up to : Borwein, J.M. ; Borwein, P.B. ( 1988 ). `` Ramanujan and Pi ''. Scientific American. 256 ( 2 ) : 112 -- 117. Bibcode : 1988SciAm. 258b. 112B. doi : 10.1038 / scientificamerican0288 - 112. Arndt & Haenel 2006, pp. 15 -- 17, 70 -- 72, 104, 156, 192 -- 197, 201 -- 202 Jump up ^ Arndt & Haenel 2006, pp. 69 -- 72 Jump up ^ Borwein, J.M. ; Borwein, P.B. ; Dilcher, K. ( 1989 ). `` Pi, Euler Numbers, and Asymptotic Expansions ''. American Mathematical Monthly. 96 ( 8 ) : 681 -- 687. doi : 10.2307 / 2324715. Jump up ^ Arndt & Haenel 2006, p. 223, ( formula 16.10 ). Note that ( n − 1 ) n ( n + 1 ) = n − n. Wells, David ( 1997 ). The Penguin Dictionary of Curious and Interesting Numbers ( revised ed. ). Penguin. p. 35. ISBN 978 - 0 - 140 - 26149 - 3. ^ Jump up to : Posamentier & Lehmann 2004, pp. 284 Jump up ^ Lambert, Johann, `` Mémoire sur quelques propriétés remarquables des quantités transcendantes circulaires et logarithmiques '', reprinted in Berggren, Borwein & Borwein 1997, pp. 129 -- 140 Jump up ^ Arndt & Haenel 2006, p. 196. Jump up ^ Hardy and Wright 1938 and 2000 : 177 footnote § 11.13 -- 14 references Lindemann 's proof as appearing at Math. Ann. 20 ( 1882 ), 213 -- 225. Jump up ^ cf Hardy and Wright 1938 and 2000 : 177 footnote § 11.13 -- 14. The proofs that e and π are transcendental can be found on pp. 170 -- 176. They cite two sources of the proofs at Landau 1927 or Perron 1910 ; see the `` List of Books '' at pp. 417 -- 419 for full citations. Jump up ^ Oughtred, William ( 1652 ). Theorematum in libris Archimedis de sphaera et cylindro declarario ( in Latin ). Excudebat L. Lichfield, Veneunt apud T. Robinson. δ. π : : semidiameter. semiperipheria ^ Jump up to : Cajori, Florian ( 2007 ). A History of Mathematical Notations : Vol. II. Cosimo, Inc. pp. 8 -- 13. ISBN 9781602067141. the ratio of the length of a circle to its diameter was represented in the fractional form by the use of two letters... J.A. Segner... in 1767, he represented 3.14159... by δ : π, as did Oughtred more than a century earlier ^ Jump up to : Smith, David E. ( 1958 ). History of Mathematics. Courier Corporation. p. 312. ISBN 9780486204307. Jump up ^ Archibald, R.C. ( 1921 ). `` Historical Notes on the Relation e − ( π / 2 ) = i i ( displaystyle e ^ ( - ( pi / 2 ) ) = i ^ ( i ) ) ``. The American Mathematical Monthly. 28 ( 3 ) : 121. doi : 10.2307 / 2972388. JSTOR 2972388. It is noticeable that these letters are never used separately, that is, π is not used for ' Semiperipheria ' ^ Jump up to : Arndt & Haenel 2006, p. 166 Jump up ^ See, for example, Oughtred, William ( 1648 ). Clavis Mathematicæ ( The key to mathematics ) ( in Latin ). London, England : Thomas Harper. p. 69. ( English translation : Oughtred, William ( 1694 ). Key of the Mathematics. J. Salusbury. ) Jump up ^ Barrow, Isaac ( 1860 ). `` Lecture XXIV ''. In Whewell, William. The mathematical works of Isaac Barrow. ( in Latin ). Harvard University. Cambridge University press. p. 381. Jump up ^ Gregorii, Davidis ( 1695 ). `` Davidis Gregorii M.D. Astronomiae Professoris Sauiliani & S.R.S. Catenaria, Ad Reverendum Virum D. Henricum Aldrich S.T.T. Decanum Aedis Christi Oxoniae ''. Philosophical Transactions ( in Latin ). 19 : 652. doi : 10.2307 / 102382. JSTOR 102382. Jump up ^ Jones, William ( 1706 ). Synopsis Palmariorum Matheseos : or, a New Introduction to the Mathematics. pp. 243, 263. Jump up ^ Arndt & Haenel 2006, p. 165. A facsimile of Jones ' text is in Berggren, Borwein & Borwein 1997, pp. 108 -- 109 Jump up ^ See Schepler 1950, p. 220 : William Oughtred used the letter π to represent the periphery ( that is, the circumference ) of a circle. Jump up ^ Segner, Joannes Andreas ( 1756 ). Cursus Mathematicus ( in Latin ). Halae Magdeburgicae. p. 282. Jump up ^ Euler, Leonhard ( 1727 ). `` Tentamen explicationis phaenomenorum aeris '' ( PDF ). Commentarii academiae scientiarum imperialis Petropolitana ( in Latin ). 2 : 351. E007. Sumatur pro ratione radii ad peripheriem, I : π English translation by Ian Bruce : `` π is taken for the ratio of the radius to the periphery ( note that in this work, Euler 's π is double our π. ) '' Jump up ^ Euler, Leonhard ( 1747 ). Henry, Charles, ed. Lettres inédites d'Euler à d'Alembert. Bullettino di Bibliografia e di Storia delle Scienze Matematiche e Fisiche ( in French ). 19 ( published 1886 ). p. 139. E858. Car, soit π la circonference d'un cercle, dout le rayon est = 1 English translation in Cajori, Florian ( 1913 ). `` History of the Exponential and Logarithmic Concepts : ''. The American Mathematical Monthly. 20 ( 3 ) : 76. doi : 10.2307 / 2973441. JSTOR 2973441. Letting π be the circumference ( ! ) of a circle of unit radius Jump up ^ Euler, Leonhard ( 1736 ). `` Ch. 3 Prop. 34 Cor. 1 ''. Mechanica sive motus scientia analytice exposita. ( cum tabulis ) ( in Latin ). 1. Academiae scientiarum Petropoli. p. 113. E015. Denotet 1 : π rationem diametri ad peripheriam English translation by Ian Bruce : `` Let 1 : π denote the ratio of the diameter to the circumference '' Jump up ^ Euler, Leonhard ( 1707 - 1783 ) ( 1922 ). Leonhardi Euleri opera omnia. 1, Opera mathematica. Volumen VIII, Leonhardi Euleri introductio in analysin infinitorum. Tomus primus / ediderunt Adolf Krazer et Ferdinand Rudio ( in Latin ). Lipsae : B.G. Teubneri. pp. 133 -- 134. E101. Jump up ^ Arndt & Haenel 2006, p. 205 Jump up ^ Arndt & Haenel 2006, p. 197. See also Reitwiesner 1950. Jump up ^ Arndt & Haenel 2006, p. 197 Jump up ^ Arndt & Haenel 2006, pp. 15 -- 17 Jump up ^ Arndt & Haenel 2006, p. 131 Jump up ^ Arndt & Haenel 2006, pp. 132, 140 ^ Jump up to : Arndt & Haenel 2006, p. 87 Jump up ^ Arndt & Haenel 2006, pp. 111 ( 5 times ) ; pp. 113 -- 114 ( 4 times ). See Borwein & Borwein 1987 for details of algorithms. ^ Jump up to : Bailey, David H. ( 16 May 2003 ). `` Some Background on Kanada 's Recent Pi Calculation '' ( PDF ). Archived ( PDF ) from the original on 15 April 2012. Retrieved 12 April 2012. Jump up ^ James Grime, Pi and the size of the Universe, Numberphile Jump up ^ Arndt & Haenel 2006, pp. 17 -- 19 Jump up ^ Schudel, Matt ( 25 March 2009 ). `` John W. Wrench, Jr. : Mathematician Had a Taste for Pi ''. The Washington Post. p. B5. Jump up ^ Connor, Steve ( 8 January 2010 ). `` The Big Question : How close have we come to knowing the precise value of pi ? ''. The Independent. London. Archived from the original on 2 April 2012. Retrieved 14 April 2012. Jump up ^ Arndt & Haenel 2006, p. 18 Jump up ^ Arndt & Haenel 2006, pp. 103 -- 104 Jump up ^ Arndt & Haenel 2006, p. 104 Jump up ^ Arndt & Haenel 2006, pp. 104, 206 Jump up ^ Arndt & Haenel 2006, pp. 110 -- 111 Jump up ^ Eymard & Lafon 1999, p. 254 Jump up ^ Arndt & Haenel 2006, pp. 110 -- 111, 206 Bellard, Fabrice, `` Computation of 2700 billion decimal digits of Pi using a Desktop Computer '' Archived 18 May 2011 at the Wayback Machine., 11 Feb 2010. ^ Jump up to : `` Round 2... 10 Trillion Digits of Pi '' Archived 1 January 2014 at the Wayback Machine., NumberWorld.org, 17 Oct 2011. Retrieved 30 May 2012. Jump up ^ PSLQ means Partial Sum of Least Squares. Jump up ^ Plouffe, Simon ( April 2006 ). `` Identities inspired by Ramanujan 's Notebooks ( part 2 ) '' ( PDF ). Archived ( PDF ) from the original on 14 January 2012. Retrieved 10 April 2009. Jump up ^ Arndt & Haenel 2006, p. 39 Jump up ^ Ramaley, J.F. ( October 1969 ). `` Buffon 's Noodle Problem ''. The American Mathematical Monthly. 76 ( 8 ) : 916 -- 918. doi : 10.2307 / 2317945. JSTOR 2317945. Jump up ^ Arndt & Haenel 2006, pp. 39 -- 40 Posamentier & Lehmann 2004, p. 105 Jump up ^ Grünbaum, B. ( 1960 ), `` Projection Constants '', Trans. Amer. Math. Soc., 95 : 451 -- 465, doi : 10.1090 / s0002 - 9947 - 1960 - 0114110 - 9 Jump up ^ Arndt & Haenel 2006, pp. 43 Posamentier & Lehmann 2004, pp. 105 -- 108 ^ Jump up to : Arndt & Haenel 2006, pp. 77 -- 84 ^ Jump up to : Gibbons, Jeremy, `` Unbounded Spigot Algorithms for the Digits of Pi '' Archived 2 December 2013 at the Wayback Machine., 2005. Gibbons produced an improved version of Wagon 's algorithm. ^ Jump up to : Arndt & Haenel 2006, p. 77 Jump up ^ Rabinowitz, Stanley ; Wagon, Stan ( March 1995 ). `` A spigot algorithm for the digits of Pi ''. American Mathematical Monthly. 102 ( 3 ) : 195 -- 203. doi : 10.2307 / 2975006. A computer program has been created that implements Wagon 's spigot algorithm in only 120 characters of software. ^ Jump up to : Arndt & Haenel 2006, pp. 117, 126 -- 128 Jump up ^ Bailey, David H. ; Borwein, Peter B. ; Plouffe, Simon ( April 1997 ). `` On the Rapid Computation of Various Polylogarithmic Constants '' ( PDF ). Mathematics of Computation. 66 ( 218 ) : 903 -- 913. doi : 10.1090 / S0025 - 5718 - 97 - 00856 - 9. Archived ( PDF ) from the original on 22 July 2012. Jump up ^ Arndt & Haenel 2006, p. 128. Plouffe did create a decimal digit extraction algorithm, but it is slower than full, direct computation of all preceding digits. Jump up ^ Arndt & Haenel 2006, p. 20 Bellards formula in : Bellard, Fabrice. `` A new formula to compute the n binary digit of pi ''. Archived from the original on 12 September 2007. Retrieved 27 October 2007. Jump up ^ Palmer, Jason ( 16 September 2010 ). `` Pi record smashed as team finds two - quadrillionth digit ''. BBC News. Archived from the original on 17 March 2011. Retrieved 26 March 2011. Jump up ^ Bronshteĭn & Semendiaev 1971, pp. 200, 209 Jump up ^ Weisstein, Eric W. `` Semicircle ''. MathWorld. ^ Jump up to : Ayers 1964, p. 60 ^ Jump up to : Bronshteĭn & Semendiaev 1971, pp. 210 -- 211 Jump up ^ Hilbert, David ; Courant, Richard ( 1966 ), Methods of mathematical physics, volume 1, Wiley, pp. 286 -- 290 Jump up ^ Dym, H. ; McKean, H.P. ( 1972 ), Fourier series and integrals, Academic Press, p. 47 Jump up ^ Chavel, Isaac ( 2001 ), Isoperimetric inequalities, Cambridge University Press Jump up ^ Talenti, Giorgio, `` Best constant in Sobolev inequality '', Annali di Matematica Pura ed Applicata, 110 ( 1 ) : 353 -- 372, doi : 10.1007 / BF02418013, ISSN 1618 - 1891, archived from the original on 29 January 2017 Jump up ^ L. Esposito ; C. Nitsch ; C. Trombetti, Best constants in Poincaré inequalities for convex domains, arXiv : 1110.2960 Jump up ^ M Del Pino ; J Dolbeault ( 2002 ), `` Best constants for Gagliardo -- Nirenberg inequalities and applications to nonlinear diffusions '', Journal de Mathématiques Pures et Appliquées, 81 ( 9 ) : 847 -- 875, CiteSeerX 10.1. 1.57. 7077, doi : 10.1016 / s0021 - 7824 ( 02 ) 01266 - 7 Jump up ^ Payne, L.E. ; Weinberger, H.F. ( 1960 ), `` An optimal Poincaré inequality for convex domains '', Archive for Rational Mechanics and Analysis : 286 -- 292, ISSN 0003 - 9527 Jump up ^ Gerald Folland ( 1989 ), Harmonic analysis in phase space, Princeton University Press, p. 5 Jump up ^ Howe 1980 Jump up ^ Feller, W. An Introduction to Probability Theory and Its Applications, Vol. 1, Wiley, 1968, pp. 174 -- 190. ^ Jump up to : Bronshteĭn & Semendiaev 1971, pp. 106 -- 107, 744, 748 Jump up ^ H Dym ; HP McKean ( 1972 ), Fourier series and integrals, Academic Press ; Section 2.7 Jump up ^ Elias Stein ; Guido Weiss ( 1971 ), Fourier analysis on Euclidean spaces, Princeton University Press, p. 6 ; Theorem 1.13. Jump up ^ V. Ovsienko ; S. Tabachnikov ( 2004 ), Projective Differential Geometry Old and New : From the Schwarzian Derivative to the Cohomology of Diffeomorphism Groups, Cambridge Tracts in Mathematics, Cambridge University Press, ISBN 978 - 0521831864 : Section 1.3 Jump up ^ Michael Spivak ( 1999 ), A comprehensive introduction to differential geometry, 3, Publish or Perish Press ; Chapter 6. Jump up ^ Kobayashi, Shoshichi ; Nomizu, Katsumi ( 1996 ), Foundations of Differential Geometry, Vol. 2 ( New ed. ), Wiley Interscience, p. 293 ; Chapter XII Characteristic classes Jump up ^ H.M. Schey ( 1996 ) Div, Grad, Curl, and All That : An Informal Text on Vector Calculus, ISBN 0 - 393 - 96997 - 5. Jump up ^ Yeo, Adrian, The pleasures of pi, e and other interesting numbers, World Scientific Pub., 2006, p. 21, ISBN 978 - 981 - 270 - 078 - 0. Ehlers, Jürgen, Einstein 's Field Equations and Their Physical Implications, Springer, 2000, p 7, ISBN 978 - 3 - 540 - 67073 - 5. Jump up ^ Lars Ahlfors ( 1966 ), Complex analysis, McGraw - Hill, p. 115 Jump up ^ Weisstein, Eric W. `` Cauchy Integral Formula ''. MathWorld. Jump up ^ Joglekar, S.D., Mathematical Physics, Universities Press, 2005, p. 166, ISBN 978 - 81 - 7371 - 422 - 1. Jump up ^ Bronshteĭn & Semendiaev 1971, pp. 191 -- 192 Jump up ^ Emil Artin ( 1964 ), The gamma function, Athena series ; selected topics in mathematics ( 1st ed. ), Holt, Rinehart and Winston Jump up ^ Lawrence Evans ( 1997 ), Partial differential equations, AMS, p. 615. Jump up ^ Bronshteĭn & Semendiaev 1971, p. 190 Jump up ^ Benjamin Nill ; Andreas Paffenholz ( 2014 ), `` On the equality case in Erhart 's volume conjecture '', Advances in Geometry, 14 ( 4 ) : 579 -- 586, arXiv : 1205.1270, doi : 10.1515 / advgeom - 2014 - 0001, ISSN 1615 - 7168 Jump up ^ Arndt & Haenel 2006, pp. 41 -- 43 Jump up ^ This theorem was proved by Ernesto Cesàro in 1881. For a more rigorous proof than the intuitive and informal one given here, see Hardy, G.H., An Introduction to the Theory of Numbers, Oxford University Press, 2008, ISBN 978 - 0 - 19 - 921986 - 5, theorem 332. Jump up ^ Ogilvy, C.S. ; Anderson, J.T., Excursions in Number Theory, Dover Publications Inc., 1988, pp. 29 -- 35, ISBN 0 - 486 - 25778 - 9. Jump up ^ Arndt & Haenel 2006, p. 43 Jump up ^ Vladimir Platonov ; Andrei Rapinchuk ( 1994 ), Algebraic groups and number theory, Academic Press, pp. 262&endash ; 265 Jump up ^ Sondow, J. ( 1994 ), `` Analytic Continuation of Riemann 's Zeta Function and Values at Negative Integers via Euler 's Transformation of Series '', Proc. Amer. Math. Soc., 120 : 421 -- 424, CiteSeerX 10.1. 1.352. 5774, doi : 10.1090 / s0002 - 9939 - 1994 - 1172954 - 7 Jump up ^ T. Friedmann ; C.R. Hagen ( 2015 ). `` Quantum mechanical derivation of the Wallis formula for pi ''. Journal of Mathematical Physics. 56 ( 11 ). arXiv : 1510.07813. doi : 10.1063 / 1.4930800. Jump up ^ Tate, John T. ( 1950 ), `` Fourier analysis in number fields, and Hecke 's zeta - functions '', Algebraic Number Theory ( Proc. Instructional Conf., Brighton, 1965 ), Thompson, Washington, D.C., pp. 305 -- 347, ISBN 978 - 0 - 9502734 - 2 - 6, MR 0217026 Jump up ^ H Dym ; HP McKean ( 1972 ), Fourier series and integrals, Academic Press ; Chapter 4 ^ Jump up to : Mumford, David ( 1983 ), Tata Lectures on Theta I, Boston : Birkhauser, pp. 1 -- 117, ISBN 3 - 7643 - 3109 - 7 Jump up ^ Sidney Port ; Charles Stone ( 1978 ), Brownian motion and classical potential theory, Academic Press, p. 29 Jump up ^ * Titchmarsh, E ( 1948 ), Introduction to the theory of Fourier integrals ( 2nd ed. ), Oxford University : Clarendon Press ( published 1986 ), ISBN 978 - 0 - 8284 - 0324 - 5. Jump up ^ Stein, Elias ( 1970 ), Singular integrals and differentiability properties of functions, Princeton University Press ; Chapter II. ^ Jump up to : Klebanoff, Aaron ( 2001 ). `` Pi in the Mandelbrot set '' ( PDF ). Fractals. 9 ( 4 ) : 393 -- 402. doi : 10.1142 / S0218348X01000828. Archived from the original ( PDF ) on 6 April 2012. Retrieved 14 April 2012. Jump up ^ Peitgen, Heinz - Otto, Chaos and fractals : new frontiers of science, Springer, 2004, pp. 801 -- 803, ISBN 978 - 0 - 387 - 20229 - 7. Jump up ^ Halliday, David ; Resnick, Robert ; Walker, Jearl, Fundamentals of Physics, 5th Ed., John Wiley & Sons, 1997, p. 381, ISBN 0 - 471 - 14854 - 7. Jump up ^ Imamura, James M ( 17 August 2005 ). `` Heisenberg Uncertainty Principle ''. University of Oregon. Archived from the original on 12 October 2007. Retrieved 9 September 2007. Jump up ^ C. Itzykson, J-B. Zuber, Quantum Field Theory, McGraw - Hill, 1980. Jump up ^ Low, Peter, Classical Theory of Structures Based on the Differential Equation, CUP Archive, 1971, pp. 116 -- 118, ISBN 978 - 0 - 521 - 08089 - 7. Jump up ^ Batchelor, G.K., An Introduction to Fluid Dynamics, Cambridge University Press, 1967, p. 233, ISBN 0 - 521 - 66396 - 2. Jump up ^ Hans - Henrik Stølum ( 22 March 1996 ). `` River Meandering as a Self - Organization Process ''. Science. 271 ( 5256 ) : 1710 -- 1713. Bibcode : 1996Sci... 271.1710 S. doi : 10.1126 / science. 271.5256. 1710. Jump up ^ Posamentier & Lehmann 2004, pp. 140 -- 141 ^ Jump up to : Arndt & Haenel 2006, pp. 44 -- 45 Jump up ^ `` Most Pi Places Memorized '' Archived 14 February 2016 at the Wayback Machine., Guinness World Records. Jump up ^ Otake, Tomoko ( 17 December 2006 ). `` How can anyone remember 100,000 numbers ? ''. The Japan Times. Archived from the original on 18 August 2013. Retrieved 27 October 2007. Jump up ^ Raz, A. ; Packard, M.G. ( 2009 ). `` A slice of pi : An exploratory neuroimaging study of digit encoding and retrieval in a superior memorist ''. Neurocase. 15 : 361 -- 372. doi : 10.1080 / 13554790902776896. PMC 4323087. PMID 19585350. Jump up ^ Keith, Mike. `` Cadaeic Cadenza Notes & Commentary ''. Archived from the original on 18 January 2009. Retrieved 29 July 2009. Jump up ^ Keith, Michael ; Diana Keith ( February 17, 2010 ). Not A Wake : A dream embodying ( pi ) 's digits fully for 10000 decimals. Vinculum Press. ISBN 978 - 0963009715. Jump up ^ For instance, Pickover calls π `` the most famous mathematical constant of all time '', and Peterson writes, `` Of all known mathematical constants, however, pi continues to attract the most attention '', citing the Givenchy π perfume, Pi ( film ), and Pi Day as examples. See Pickover, Clifford A. ( 1995 ), Keys to Infinity, Wiley & Sons, p. 59, ISBN 9780471118572 ; Peterson, Ivars ( 2002 ), Mathematical Treks : From Surreal Numbers to Magic Circles, MAA spectrum, Mathematical Association of America, p. 17, ISBN 9780883855379, archived from the original on 29 November 2016 Jump up ^ BBC documentary `` The Story of Maths '', second part Archived 23 December 2014 at the Wayback Machine., showing a visualization of the historically first exact formula, starting at 35 min and 20 sec into the second part of the documentary. Jump up ^ Posamentier & Lehmann 2004, p. 118 Arndt & Haenel 2006, p. 50 Jump up ^ Arndt & Haenel 2006, p. 14. This part of the story was omitted from the film adaptation of the novel. Jump up ^ Gill, Andy ( 4 November 2005 ). `` Review of Aerial ''. The Independent. Archived from the original on 15 October 2006. the almost autistic satisfaction of the obsessive - compulsive mathematician fascinated by ' Pi ' ( which affords the opportunity to hear Bush slowly sing vast chunks of the number in question, several dozen digits long ) Jump up ^ Pi Day activities. Jump up ^ MIT cheers Archived 19 January 2009 at the Wayback Machine... Retrieved 12 April 2012. Jump up ^ `` Happy Pi Day ! Watch these stunning videos of kids reciting 3.14 ''. USAToday.com. 14 March 2015. Archived from the original on 15 March 2015. Retrieved 14 March 2015. Jump up ^ `` Google 's strange bids for Nortel patents ''. FinancialPost.com. Reuters. 5 July 2011. Archived from the original on 9 August 2011. Retrieved 16 August 2011. Jump up ^ Eagle, Albert ( 1958 ). The Elliptic Functions as They Should be : An Account, with Applications, of the Functions in a New Canonical Form. Galloway and Porter, Ltd. p. ix. Jump up ^ Sequence A019692, Jump up ^ Abbott, Stephen ( April 2012 ). `` My Conversion to Tauism '' ( PDF ). Math Horizons. 19 ( 4 ) : 34. doi : 10.4169 / mathhorizons. 19.4. 34. Archived ( PDF ) from the original on 28 September 2013. Jump up ^ Palais, Robert ( 2001 ). `` π Is Wrong ! '' ( PDF ). The Mathematical Intelligencer. 23 ( 3 ) : 7 -- 8. doi : 10.1007 / BF03026846. Archived ( PDF ) from the original on 22 June 2012. Jump up ^ Tau Day : Why you should eat twice the pie -- Light Years -- CNN.com Blogs Archived 12 January 2013 at the Wayback Machine. Jump up ^ `` Life of pi in no danger -- Experts cold - shoulder campaign to replace with tau ''. Telegraph India. 30 June 2011. Archived from the original on 13 July 2013. Jump up ^ Arndt & Haenel 2006, pp. 211 -- 212 Posamentier & Lehmann 2004, pp. 36 -- 37 Hallerberg, Arthur ( May 1977 ). `` Indiana 's squared circle ''. Mathematics Magazine. 50 ( 3 ) : 136 -- 140. doi : 10.2307 / 2689499. JSTOR 2689499. Jump up ^ Knuth, Donald ( 3 October 1990 ). `` The Future of TeX and Metafont '' ( PDF ). TeX Mag. 5 ( 1 ) : 145. Archived ( PDF ) from the original on 13 April 2016. Retrieved 17 February 2017. References Arndt, Jörg ; Haenel, Christoph ( 2006 ). Pi Unleashed. Springer - Verlag. ISBN 978 - 3 - 540 - 66572 - 4. Retrieved 2013 - 06 - 05. English translation by Catriona and David Lischka. Ayers, Frank ( 1964 ). Calculus. McGraw - Hill. ISBN 978 - 0 - 070 - 02653 - 7. Beckmann, Peter ( 1989 ) ( 1974 ). History of Pi. St. Martin 's Press. ISBN 978 - 0 - 88029 - 418 - 8. Berggren, Lennart ; Borwein, Jonathan ; Borwein, Peter ( 1997 ). Pi : a Source Book. Springer - Verlag. ISBN 978 - 0 - 387 - 20571 - 7. Borwein, Jonathan ; Borwein, Peter ( 1987 ). Pi and the AGM : a Study in Analytic Number Theory and Computational Complexity. Wiley. ISBN 978 - 0 - 471 - 31515 - 5. Boyer, Carl B. ; Merzbach, Uta C. ( 1991 ). A History of Mathematics ( 2 ed. ). Wiley. ISBN 978 - 0 - 471 - 54397 - 8. Bronshteĭn, Ilia ; Semendiaev, K.A. ( 1971 ). A Guide Book to Mathematics. Verlag Harri Deutsch. ISBN 978 - 3 - 871 - 44095 - 3. Eymard, Pierre ; Lafon, Jean Pierre ( 1999 ). The Number Pi. American Mathematical Society. ISBN 978 - 0 - 8218 - 3246 - 2., English translation by Stephen Wilson. Howe, Roger ( 1980 ), `` On the role of the Heisenberg group in harmonic analysis '', Bulletin of the American Mathematical Society, 3 ( 2 ), doi : 10.1090 / S0273 - 0979 - 1980 - 14825 - 9, MR 0578375. Joseph, George Gheverghese ( 1991 ). The Crest of the Peacock : Non-European Roots of Mathematics. Princeton University Press. ISBN 978 - 0 - 691 - 13526 - 7. Retrieved 2013 - 06 - 05. Posamentier, Alfred S. ; Lehmann, Ingmar ( 2004 ). Pi : A Biography of the World 's Most Mysterious Number. Prometheus Books. ISBN 978 - 1 - 59102 - 200 - 8. Reitwiesner, George ( 1950 ). `` An ENIAC Determination of pi and e to 2000 Decimal Places ''. Mathematical Tables and Other Aids to Computation. 4 ( 29 ) : 11 -- 15. doi : 10.2307 / 2002695. Roy, Ranjan ( 1990 ). `` The Discovery of the Series Formula for pi by Leibniz, Gregory, and Nilakantha ''. Mathematics Magazine. 63 ( 5 ) : 291 -- 306. doi : 10.2307 / 2690896. Schepler, H.C. ( 1950 ). `` The Chronology of Pi ''. Mathematics Magazine. Mathematical Association of America. 23 ( 3 ) : 165 -- 170 ( Jan / Feb ), 216 -- 228 ( Mar / Apr ), and 279 -- 283 ( May / Jun ). doi : 10.2307 / 3029284... issue 3 Jan / Feb, issue 4 Mar / Apr, issue 5 May / Jun Thompson, William ( 1894 ), `` Isoperimetrical problems '', Nature series : Popular lectures and addresses, MacMillan and Company, II : 571 -- 592 Further reading Blatner, David ( 1999 ). The Joy of Pi. Walker & Company. ISBN 978 - 0 - 8027 - 7562 - 7. Borwein, Jonathan ; Borwein, Peter ( 1984 ). `` The Arithmetic - Geometric Mean and Fast Computation of Elementary Functions '' ( PDF ). SIAM Review. 26 ( 3 ) : 351 -- 365. doi : 10.1137 / 1026073. Borwein, Jonathan ; Borwein, Peter ; Bailey, David H. ( 1989 ). `` Ramanujan, Modular Equations, and Approximations to Pi or How to Compute One Billion Digits of Pi ''. The American Mathematical Monthly. 96 ( 3 ) : 201 -- 219. doi : 10.2307 / 2325206. JSTOR 2325206. Chudnovsky, David V. and Chudnovsky, Gregory V., `` Approximations and Complex Multiplication According to Ramanujan '', in Ramanujan Revisited ( G.E. Andrews et al. Eds ), Academic Press, 1988, pp 375 -- 396, 468 -- 472 Cox, David A., `` The Arithmetic - Geometric Mean of Gauss '', L ' Ensignement Mathematique, 30 ( 1984 ) 275 -- 330 Delahaye, Jean - Paul, `` Le Fascinant Nombre Pi '', Paris : Bibliothèque Pour la Science ( 1997 ) ISBN 2902918259 Engels, Hermann ( 1977 ). `` Quadrature of the Circle in Ancient Egypt ''. Historia Mathematica. 4 ( 2 ) : 137 -- 140. doi : 10.1016 / 0315 - 0860 ( 77 ) 90104 - 5. Euler, Leonhard, `` On the Use of the Discovered Fractions to Sum Infinite Series '', in Introduction to Analysis of the Infinite. Book I, translated from the Latin by J.D. Blanton, Springer - Verlag, 1964, pp 137 -- 153 Hardy, G.H. and Wright E.M., An Introduction to the Theory of Numbers first published 1938, fifth Edition 1979 with additions 2000, Clarendon Press, Oxford U.K. Heath, T.L., The Works of Archimedes, Cambridge, 1897 ; reprinted in The Works of Archimedes with The Method of Archimedes, Dover, 1953, pp 91 -- 98 Huygens, Christiaan, `` De Circuli Magnitudine Inventa '', Christiani Hugenii Opera Varia I, Leiden 1724, pp 384 -- 388 Lay - Yong, Lam ; Tian - Se, Ang ( 1986 ). `` Circle Measurements in Ancient China ''. Historia Mathematica. 13 ( 4 ) : 325 -- 340. doi : 10.1016 / 0315 - 0860 ( 86 ) 90055 - 8. Lindemann, Ferdinand ( 1882 ). `` Ueber die Zahl pi ''. Mathematische Annalen. 20 ( 2 ) : 213 -- 225. doi : 10.1007 / bf01446522. Archived from the original on 22 January 2015. Matar, K. Mukunda ; Rajagonal, C. ( 1944 ). `` On the Hindu Quadrature of the Circle '' ( Appendix by K. Balagangadharan ) ``. 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Reprinted in vol. 1 ( pp 357 -- 478 ) of Opera Mathematica, Oxford 1693 Zebrowski, Ernest, A History of the Circle : Mathematical Reasoning and the Physical Universe, Rutgers University Press, 1999, ISBN 978 - 0 - 8135 - 2898 - 4 External links Wikimedia Commons has media related to Pi. 10 million decimal places `` Pi '' at Wolfram Mathworld Representations of Pi at Wolfram Alpha Demonstration by Lambert ( 1761 ) of irrationality of π, online and analyzed BibNum ( PDF ). π Search Engine 2 billion searchable digits of π, e and √ 2 hide Irrational numbers Prime ( ρ ) Euler -- Mascheroni ( γ ) Logarithm of 2 Twelfth root of two ( √ 2 ) Apéry 's ( ζ ( 3 ) ) Sophomore 's dream Plastic ( ρ ) Square root of 2 ( √ 2 ) Erdős -- Borwein ( E ) Golden ratio ( φ ) Square root of 3 ( √ 3 ) Square root of 5 ( √ 5 ) Silver ratio ( δ ) Second Feigenbaum ( α ) E ( e ) Pi ( π ) First Feigenbaum ( δ ) Schizophrenic Transcendental Trigonometric GND : 4174646 - 6 LCCN : sh85101712 NDL : 00562015 Retrieved from `` https://en.wikipedia.org/w/index.php?title=Pi&oldid=843547734 '' Categories : Pi Complex analysis Mathematical series Real transcendental numbers Hidden categories : Webarchive template wayback links CS1 German - language sources ( de ) CS1 French - language sources ( fr ) CS1 Latin - language sources ( la ) Wikipedia indefinitely semi-protected pages Use dmy dates from July 2012 Featured articles Wikipedia articles with GND identifiers Wikipedia articles with LCCN identifiers Wikipedia articles with NDL identifiers Talk View source Contents About Wikipedia Wikiquote Afrikaans Alemannisch አማርኛ Aragonés অসমীয়া Asturianu Azərbaycanca تۆرکجه বাংলা Bahasa Banjar Bân - lâm - gú Башҡортса Беларуская Беларуская ( тарашкевіца ) ‎ Български Bosanski Brezhoneg Буряад Català Чӑвашла Cebuano Čeština Cymraeg Dansk Deutsch Dolnoserbski Eesti Ελληνικά Emiliàn e rumagnòl Español Esperanto Estremeñu Euskara فارسی Føroyskt Français Frysk Furlan Gaeilge Galego 贛 語 ગુજરાતી Хальмг 한국어 Hawaiʻi Հայերեն हिन्दी Hornjoserbsce Hrvatski Ido Bahasa Indonesia Interlingua Ирон Íslenska Italiano עברית Basa Jawa ಕನ್ನಡ ქართული Қазақша Kiswahili Kreyòl ayisyen Kurdî Лезги Latina Latviešu Lëtzebuergesch Lietuvių Limburgs Lumbaart Magyar Македонски Malagasy മലയാളം मराठी مصرى Bahasa Melayu Mìng - dĕ̤ng - ngṳ̄ Монгол မြန်မာဘာသာ Na Vosa Vakaviti Nederlands नेपाल भाषा 日本 語 Norsk Norsk nynorsk Occitan ଓଡ଼ିଆ Oʻzbekcha / ўзбекча ਪੰਜਾਬੀ Pälzisch پنجابی Patois Picard Piemontèis Plattdüütsch Polski Português Ripoarisch Română Runa Simi Русиньскый Русский Саха тыла Sardu Scots Shqip Sicilianu සිංහල Simple English Slovenčina Slovenščina Ślůnski Soomaaliga کوردی Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska Tagalog தமிழ் Tarandíne Татарча / tatarça తెలుగు ไทย Türkçe Українська اردو Vèneto Tiếng Việt 文言 Winaray 吴语 ייִדיש Yorùbá 粵語 Zeêuws Žemaitėška 中文 131 more Edit links This page was last edited on 29 May 2018, at 21 : 14 ( UTC ). 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Question: where does the symbol for pi come from
Answer: the lowercase Greek letter π, sometimes spelled out as pi, and derived from the first letter of the Greek word perimetros, meaning circumference