Riemann zeta function
The Riemann zeta function or Euler–Riemann zeta function, denoted by the Greek letter (zeta), is a mathematical function of a complex variable defined as formula_1 for formula_2 and its analytic continuation elsewhere. The Riemann zeta function plays a pivotal role in analytic number theory, and has applications in physics, probability theory, and applied statistics. Leonhard Euler first introduced and studied the function over the reals in the first half of the eighteenth century. Bernhard Riemann's 1859 article "On the Number of Primes Less Than a Given Magnitude" extended the Euler definition to a complex variable , proved its meromorphic continuation and functional equation, and established a relation between its zeros and the distribution of prime numbers. This paper also contained the Riemann hypothesis, a conjecture about the distribution of complex zeros of the Riemann zeta function that is considered by many mathematicians to be the most important unsolved problem in pure mathematics. The values of the Riemann zeta function at even positive integers were computed by Euler. The first of them, , provides a solution to the Basel problem. In 1979 Roger Apéry proved the irrationality of . The values at negative integer points, also found by Euler , are rational numbers and play an important role in the theory of modular forms. Many generalizations of the Riemann zeta function, such as Dirichlet series, Dirichlet -functions and -functions, are known. Definition. The Riemann zeta function is a function of a complex variable . (The notation , , and is used traditionally in the study of the zeta function, following Riemann.) When , the function can be written as a converging summation or integral: where is the gamma function. The Riemann zeta function is defined for other complex values via analytic continuation of the function defined for . Leonhard Euler considered the above series in  1740 for positive integer values of , and later Chebyshev extended the definition to formula_5 The above series is a prototypical Dirichlet series that converges absolutely to an analytic function for such that and diverges for all other values of . Riemann showed that the function defined by the series on the half-plane of convergence can be continued analytically to all complex values . For , the series is the harmonic series which diverges to , and formula_6 Thus the Riemann zeta function is a meromorphic function on the whole complex plane, which is holomorphic everywhere except for a simple pole at with residue  . Euler's product formula. In 1737, the connection between the zeta function and prime numbers was discovered by Euler, who proved the identity where, by definition, the left hand side is and the infinite product on the right hand side extends over all prime numbers (such expressions are called Euler products): Both sides of the Euler product formula converge for . The proof of Euler's identity uses only the formula for the geometric series and the fundamental theorem of arithmetic. Since the harmonic series, obtained when , diverges, Euler's formula (which becomes ) implies that there are infinitely many primes. The  Euler product formula can be used to calculate the asymptotic probability that randomly selected integers are set-wise coprime. Intuitively, the probability that any single number is divisible by a prime (or any integer) is . Hence the probability that numbers are all divisible by this prime is , and the probability that at least one of them is "not" is . Now, for distinct primes, these divisibility events are mutually independent because the candidate divisors are coprime (a number is divisible by coprime divisors and if and only if it is divisible by , an event which occurs with probability ). Thus the asymptotic  probability that numbers are coprime is given by a product over all primes, Riemann's functional equation. This zeta function satisfies the functional equation formula_10 where is the gamma function. This is an equality of meromorphic functions valid on the whole complex plane. The equation relates values of the Riemann zeta function at the points and , in particular relating even positive integers with odd negative integers. Owing to the zeros of the sine function, the functional equation implies that has a simple zero at each even negative integer , known as the trivial zeros of . When is an even positive integer , the product on the right is non-zero because has a simple pole, which cancels the simple zero of the sine factor. The functional equation was established by Riemann in his 1859 paper "On the Number of Primes Less Than a Given Magnitude" and used to construct the analytic continuation in the first place. An equivalent relationship had been conjectured by Euler over a hundred years earlier, in 1749, for the Dirichlet eta function (the alternating zeta function): formula_11 Incidentally, this relation gives an equation for calculating in the region 0 < < 1, i.e. formula_12 where the "η"-series is  convergent (albeit non-absolutely) in the larger half-plane (for a more detailed survey on the history of the functional equation, see e.g. Blagouchine). Riemann also found a symmetric version of the functional equation applying to the xi-function: formula_13 which satisfies: formula_14 Zeros, the critical line, and the Riemann hypothesis. The functional equation shows that the Riemann zeta function has zeros at . These are called the trivial zeros. They are trivial in the sense that their existence is relatively easy to prove, for example, from being 0 in the functional equation. The non-trivial zeros have captured far more  attention because their distribution not only is far less understood but, more importantly, their study yields important results concerning prime numbers and related objects in number theory. It is known that any non-trivial zero lies in the open strip formula_15, which is called the critical strip. The set formula_16 is called the critical line. The Riemann hypothesis, considered one of the greatest unsolved problems in mathematics, asserts that all non-trivial zeros are on the critical line. In 1989, Conrey proved that more than 40% of the non-trivial zeros of the Riemann zeta function are on the critical  line. For the Riemann zeta function on the critical line, see -function. Number of zeros in the critical strip. Let formula_17 be the number of zeros of formula_18 in the critical strip formula_19, whose imaginary parts are in the interval formula_20. Trudgian proved that, if formula_21, then The Hardy–Littlewood conjectures. In 1914, Godfrey Harold Hardy proved that has infinitely many real zeros. Hardy and John Edensor Littlewood formulated two conjectures on the density and distance between the zeros of on intervals of large positive real numbers. In the following, is the total number of real zeros and the total  number of zeros of odd order of the function lying in the interval . These two conjectures opened up new directions in the investigation of the Riemann zeta function. Zero-free region. The location of the Riemann zeta function's zeros is of great importance in number theory. The prime number theorem is equivalent to the fact that there are no zeros of the zeta function on the line. A better result that follows from an effective form of Vinogradov's mean-value theorem is that whenever formula_23 and . In 2015, Mossinghoff and Trudgian proved that zeta has no zeros in  the region for . This is the largest known zero-free region in the critical strip for formula_25. The strongest result of this kind one can hope for is the truth of the Riemann hypothesis, which would have many profound consequences in the theory of numbers. Other results. It is known that there are infinitely many zeros on the critical line. Littlewood showed that if the sequence () contains the imaginary parts of all zeros in the upper half-plane in ascending order, then The critical line theorem asserts that a positive proportion of the nontrivial zeros lies on the critical line . (The Riemann hypothesis would imply that this proportion is 1.) In the critical strip, the zero with smallest non-negative imaginary part is (). The fact that for all complex implies that the zeros of the Riemann zeta function are symmetric about the real axis. Combining this symmetry with the functional equation, furthermore, one sees that the non-trivial zeros are symmetric about the critical line . It is also known that no zeros lie on a line with real part 1. Specific values. For any positive even integer , formula_28 where is the -th Bernoulli number. For odd positive integers, no such  simple expression is known, although these values are thought to be related to the algebraic -theory of the integers; see Special values of -functions. For nonpositive integers, one has formula_29 for (using the convention that ). In particular, vanishes at the negative even integers because for all odd other than 1. These are the so-called "trivial zeros" of the zeta function. Via analytic continuation, one can show that formula_30 This gives a pretext for assigning a finite value to the divergent series 1 + 2 + 3 + 4 + ⋯, which has been used in certain contexts (Ramanujan summation ) such as string theory. Analogously, the particular value formula_31 can be viewed as assigning a finite result to the divergent series 1 + 1 + 1 + 1 + ⋯. The value formula_32 is employed in calculating kinetic boundary layer problems of linear kinetic equations. Although formula_33 diverges, its Cauchy principal value formula_34 exists and is equal to the Euler–Mascheroni constant . The demonstration of the particular value formula_35 is known as the Basel problem. The reciprocal of this sum answers the question: "What is the probability that two numbers selected at random are relatively prime?" The value formula_36 is  Apéry's constant. Taking the limit formula_37 through the real numbers, one obtains formula_38. But at complex infinity on the Riemann sphere, the zeta function has an essential singularity. Various properties. For sums involving the zeta function at integer and half-integer values, see rational zeta series. Reciprocal. The reciprocal of the zeta function may be expressed as a Dirichlet series over the Möbius function : for every complex number with real part greater than 1. There are a number of similar relations involving various well-known multiplicative functions; these are given in the article on the Dirichlet series. The Riemann  hypothesis is equivalent to the claim that this expression is valid when the real part of is greater than . Universality. The critical strip of the Riemann zeta function has the remarkable property of universality. This zeta function universality states that there exists some location on the critical strip that approximates any holomorphic function arbitrarily well. Since holomorphic functions are very general, this property is quite remarkable. The first proof of universality was provided by Sergei Mikhailovitch Voronin in 1975. More recent work has included effective versions of Voronin's theorem and extending it to Dirichlet L-functions. Estimates of the  maximum of the modulus of the zeta function. Let the functions and be defined by the equalities Here is a sufficiently large positive number, , , , . Estimating the values and from below shows, how large (in modulus) values can take on short intervals of the critical line or in small neighborhoods of points lying in the critical strip . The case was studied by Kanakanahalli Ramachandra; the case , where is a sufficiently large constant, is trivial. Anatolii Karatsuba proved, in particular, that if the values and exceed certain sufficiently small constants, then the estimates hold, where and are certain absolute constants. The argument  of the Riemann zeta function. The function is called the argument of the Riemann zeta function. Here is the increment of an arbitrary continuous branch of along the broken line joining the points , and . There are some theorems on properties of the function . Among those results are the mean value theorems for and its first integral on intervals of the real line, and also the theorem claiming that every interval for contains at least points where the function changes sign. Earlier similar results were obtained by Atle Selberg for the case Representations. Dirichlet series. An extension of the area of  convergence can be obtained by rearranging the original series. The series converges for , while converges even for . In this way, the area of convergence can be extended to for any negative integer . Mellin-type integrals. The Mellin transform of a function is defined as in the region where the integral is defined. There are various expressions for the zeta function as Mellin transform-like integrals. If the real part of is greater than one, we have where denotes the gamma function. By modifying the contour, Riemann showed that for all (where denotes the Hankel contour). We can also find expressions  which relate to prime numbers and the prime number theorem. If is the prime-counting function, then for values with . A similar Mellin transform involves the Riemann function , which counts prime powers with a weight of , so that Now These expressions can be used to prove the prime number theorem by means of the inverse Mellin transform. Riemann's prime-counting function is easier to work with, and can be recovered from it by Möbius inversion. Theta functions. The Riemann zeta function can be given by a Mellin transform in terms of Jacobi's theta function However, this integral only  converges if the real part of is greater than 1, but it can be regularized. This gives the following expression for the zeta function, which is well defined for all except 0 and 1: Laurent series. The Riemann zeta function is meromorphic with a single pole of order one at . It can therefore be expanded as a Laurent series about ; the series development is then The constants here are called the Stieltjes constants and can be defined by the limit The constant term is the Euler–Mascheroni constant. Integral. For all , , the integral relation (cf. Abel–Plana formula) holds true , which may be used for a numerical evaluation of the zeta function. Rising factorial. Another series development using the rising factorial valid for the entire complex plane is This can be used recursively to extend the Dirichlet series definition to all complex numbers. The Riemann zeta function also appears in a form similar to the Mellin transform in an integral over the Gauss–Kuzmin–Wirsing operator acting on ; that context gives rise to a series expansion in terms of the falling factorial. Hadamard product. On the basis of Weierstrass's factorization theorem, Hadamard gave the infinite product expansion where the  product is over the non-trivial zeros of and the letter again denotes the Euler–Mascheroni constant. A simpler infinite product expansion is This form clearly displays the simple pole at , the trivial zeros at −2, −4, ... due to the gamma function term in the denominator, and the non-trivial zeros at . (To ensure convergence in the latter formula, the product should be taken over "matching pairs" of zeros, i.e. the factors for a pair of zeros of the form and should be combined.) Globally convergent series. A globally convergent series for the zeta function, valid for all complex numbers  except for some integer , was conjectured by Konrad Knopp and proven by Helmut Hasse in 1930 (cf. Euler summation): The series appeared in an appendix to Hasse's paper, and was published for the second time by Jonathan Sondow in 1994. Hasse also proved the globally converging series in the same publication. Research by Iaroslav Blagouchine has found that a similar, equivalent series was published by Joseph Ser in 1926. Peter Borwein has developed an algorithm that applies Chebyshev polynomials to the Dirichlet eta function to produce a very rapidly convergent series suitable for high precision numerical calculations. Series representation  at positive integers via the primorial. Here is the primorial sequence and is Jordan's totient function. Series representation by the incomplete poly-Bernoulli numbers. The function can be represented, for , by the infinite series where , is the th branch of the Lambert -function, and is an incomplete poly-Bernoulli number. The Mellin transform of the Engel map. The function formula_69 is iterated to find the coefficients appearing in Engel expansions. The Mellin transform of the map formula_70 is related to the Riemann zeta function by the formula Numerical algorithms. A classical algorithm, in use prior to about 1930, proceeds  by applying the Euler-Maclaurin formula to obtain, for "n" and "m" positive integers, where, letting formula_73 denote the indicated Bernoulli number, and the error satisfies with "σ" = Re("s"). A modern numerical algorithm is the Odlyzko–Schönhage algorithm. Applications. The zeta function occurs in applied statistics (see Zipf's law and Zipf–Mandelbrot law). Zeta function regularization is used as one possible means of regularization of divergent series and divergent integrals in quantum field theory. In one notable example, the Riemann zeta function shows up explicitly in one method of calculating the Casimir effect. The zeta function is also  useful for the analysis of dynamical systems. In the theory of musical tunings, the zeta function can be used to find EDOs that closely approximate the intervals of the harmonic series. The values of formula_76 for increasing real formula_77 peak near integers that correspond to such EDOs. Examples include popular choices such as 12, 19, and 53. Infinite series. The zeta function evaluated at equidistant positive integers appears in infinite series representations of a number of constants. In fact the even and odd terms give the two sums and Parametrized versions of the above sums are given by and with  formula_83 and where formula_84 and formula_85 are the polygamma function and Euler's constant, as well as all of which are continuous at formula_87. Other sums include where denotes the imaginary part of a complex number. There are yet more formulas in the article Harmonic number. Generalizations. There are a number of related zeta functions that can be considered to be generalizations of the Riemann zeta function. These include the Hurwitz zeta function (the convergent series representation was given by Helmut Hasse in 1930, cf. Hurwitz zeta function), which coincides with the Riemann zeta function when (the lower limit of  summation in the Hurwitz zeta function is 0, not 1), the Dirichlet -functions and the Dedekind zeta function. For other related functions see the articles zeta function and -function. The polylogarithm is given by which coincides with the Riemann zeta function when . The Clausen function can be chosen as the real or imaginary part of . The Lerch transcendent is given by which coincides with the Riemann zeta function when and (the lower limit of summation in the Lerch transcendent is 0, not 1). The multiple zeta functions are defined by One can analytically continue these functions to the -dimensional complex  space. The special values taken by these functions at positive integer arguments are called multiple zeta values by number theorists and have been connected to many different branches in mathematics and physics.