Euler's totient function
In number theory, Euler's totient function counts the positive integers up to a given integer that are relatively prime to . It is written using the Greek letter phi as formula_1 or formula_2, and may also be called Euler's phi function. In other words, it is the number of integers in the range for which the greatest common divisor is equal to 1. The integers of this form are sometimes referred to as totatives of . For example, the totatives of are the six numbers 1, 2, 4, 5, 7 and 8. They are all relatively  prime to 9, but the other three numbers in this range, 3, 6, and 9 are not, since and . Therefore, . As another example, since for the only integer in the range from 1 to is 1 itself, and . Euler's totient function is a multiplicative function, meaning that if two numbers and are relatively prime, then . This function gives the order of the multiplicative group of integers modulo (the group of units of the ring formula_3). It is also used for defining the RSA encryption system. History, terminology, and notation. Leonhard Euler introduced the function in 1763. However, he did  not at that time choose any specific symbol to denote it. In a 1784 publication, Euler studied the function further, choosing the Greek letter to denote it: he wrote for "the multitude of numbers less than , and which have no common divisor with it". This definition varies from the current definition for the totient function at but is otherwise the same. The now-standard notation comes from Gauss's 1801 treatise "Disquisitiones Arithmeticae", although Gauss didn't use parentheses around the argument and wrote . Thus, it is often called Euler's phi function or simply the phi function. In 1879 , J. J. Sylvester coined the term totient for this function, so it is also referred to as Euler's totient function, the Euler totient, or Euler's totient. Jordan's totient is a generalization of Euler's. The cototient of is defined as . It counts the number of positive integers less than or equal to that have at least one prime factor in common with . Computing Euler's totient function. There are several formulae for computing . Euler's product formula. It states where the product is over the distinct prime numbers dividing . (For notation, see Arithmetical function.) An equivalent formulation  for formula_5, where formula_6 are the distinct primes dividing "n", is:formula_7The proof of these formulae depends on two important facts. Phi is a multiplicative function. This means that if , then . "Proof outline:" Let , , be the sets of positive integers which are coprime to and less than , , , respectively, so that , etc. Then there is a bijection between and by the Chinese remainder theorem. Value of phi for a prime power argument. If is prime and , then "Proof": Since is a prime number, the only possible values of are , and the only way to have is if is a multiple of  , that is, , and there are such multiples not greater than . Therefore, the other numbers are all relatively prime to . Proof of Euler's product formula. The fundamental theorem of arithmetic states that if there is a unique expression formula_9 where are prime numbers and each . (The case corresponds to the empty product.) Repeatedly using the multiplicative property of and the formula for gives This gives both versions of Euler's product formula. An alternative proof that does not require the multiplicative property instead uses the inclusion-exclusion principle applied to the set formula_11, excluding the sets of integers divisible by  the prime divisors. Example. In words: the distinct prime factors of 20 are 2 and 5; half of the twenty integers from 1 to 20 are divisible by 2, leaving ten; a fifth of those are divisible by 5, leaving eight numbers coprime to 20; these are: 1, 3, 7, 9, 11, 13, 17, 19. The alternative formula uses only integers:formula_13 Fourier transform. The totient is the discrete Fourier transform of the gcd, evaluated at 1. Let where for . Then The real part of this formula is For example, using formula_17 and formula_18:formula_19Unlike the Euler product and the  divisor sum formula, this one does not require knowing the factors of . However, it does involve the calculation of the greatest common divisor of and every positive integer less than , which suffices to provide the factorization anyway. Divisor sum. The property established by Gauss, that where the sum is over all positive divisors of , can be proven in several ways. (See Arithmetical function for notational conventions.) One proof is to note that is also equal to the number of possible generators of the cyclic group ; specifically, if with , then is a generator for every coprime to . Since every element of  generates a cyclic subgroup, and all subgroups are generated by precisely elements of , the formula follows. Equivalently, the formula can be derived by the same argument applied to the multiplicative group of the th roots of unity and the primitive th roots of unity. The formula can also be derived from elementary arithmetic. For example, let and consider the positive fractions up to 1 with denominator 20: Put them into lowest terms: These twenty fractions are all the positive ≤ 1 whose denominators are the divisors . The fractions with 20 as denominator are those with numerators relatively prime to 20 , namely , , , , , , , ; by definition this is fractions. Similarly, there are fractions with denominator 10, and fractions with denominator 5, etc. Thus the set of twenty fractions is split into subsets of size for each dividing 20. A similar argument applies for any "n." Möbius inversion applied to the divisor sum formula gives where is the Möbius function, the multiplicative function defined by formula_24 and formula_25 for each prime and . This formula may also be derived from the product formula by multiplying out formula_26 to get formula_27 An example:formula_28 Some values. The first 100 values are shown in the table and  graph below: In the graph at right the top line is an upper bound valid for all other than one, and attained if and only if is a prime number. A simple lower bound is formula_29, which is rather loose: in fact, the lower limit of the graph is proportional to . Euler's theorem. This states that if and are relatively prime then The special case where is prime is known as Fermat's little theorem. This follows from Lagrange's theorem and the fact that is the order of the multiplicative group of integers modulo. The RSA cryptosystem is  based on this theorem: it implies that the inverse of the function , where is the (public) encryption exponent, is the function , where , the (private) decryption exponent, is the multiplicative inverse of modulo . The difficulty of computing without knowing the factorization of is thus the difficulty of computing : this is known as the RSA problem which can be solved by factoring . The owner of the private key knows the factorization, since an RSA private key is constructed by choosing as the product of two (randomly chosen) large primes and . Only is publicly disclosed, and given the difficulty to factor large numbers  we have the guarantee that no one else knows the factorization. Other formulae. Menon's identity. In 1965 P. Kesava Menon proved where is the number of divisors of . Formulae involving the golden ratio. Schneider found a pair of identities connecting the totient function, the golden ratio and the Möbius function . In this section is the totient function, and is the golden ratio. They are: and Subtracting them gives Applying the exponential function to both sides of the preceding identity yields an infinite product formula for : The proof is based on the two formulae Generating functions. The Dirichlet series for  may be written in terms of the Riemann zeta function as: where the left-hand side converges for formula_38. The Lambert series generating function is which converges for . Both of these are proved by elementary series manipulations and the formulae for . Growth rate. In the words of Hardy & Wright, the order of is "always 'nearly '." First but as "n" goes to infinity, for all These two formulae can be proved by using little more than the formulae for and the divisor sum function . In fact, during the proof of the second formula, the inequality true for , is proved. We also  have Here is Euler's constant, , so and . Proving this does not quite require the prime number theorem. Since goes to infinity, this formula shows that In fact, more is true. and The second inequality was shown by Jean-Louis Nicolas. Ribenboim says "The method of proof is interesting, in that the inequality is shown first under the assumption that the Riemann hypothesis is true, secondly under the contrary assumption." For the average order, we have due to Arnold Walfisz, its proof exploiting estimates on exponential sums due to I. M. Vinogradov and N. M. Korobov (this is currently the  best known estimate of this type). The "Big " stands for a quantity that is bounded by a constant times the function of inside the parentheses (which is small compared to ). This result can be used to prove that the probability of two randomly chosen numbers being relatively prime is . Ratio of consecutive values. In 1950 Somayajulu proved In 1954 Schinzel and Sierpiński strengthened this, proving that the set is dense in the positive real numbers. They also proved that the set is dense in the interval (0,1). Totient numbers. A totient number is a value of Euler's totient function : that is, an for which there is at least one for which . The "valency" or "multiplicity" of a totient number is the number of solutions to this equation. A "nontotient" is a natural number which is not a totient number. Every odd integer exceeding 1 is trivially a nontotient. There are also infinitely many even nontotients, and indeed every positive integer has a multiple which is an even nontotient. The number of totient numbers up to a given limit is for a constant . If counted accordingly to multiplicity, the number of totient numbers up to a given limit is where  the error term is of order at most for any positive . It is known that the multiplicity of exceeds infinitely often for any . Ford's theorem. proved that for every integer there is a totient number of multiplicity : that is, for which the equation has exactly solutions; this result had previously been conjectured by Wacław Sierpiński, and it had been obtained as a consequence of Schinzel's hypothesis H. Indeed, each multiplicity that occurs, does so infinitely often. However, no number is known with multiplicity . Carmichael's totient function conjecture is the statement that there is no such . Applications. Cyclotomy . In the last section of the "Disquisitiones" Gauss proves that a regular -gon can be constructed with straightedge and compass if is a power of 2. If is a power of an odd prime number the formula for the totient says its totient can be a power of two only if is a first power and is a power of 2. The primes that are one more than a power of 2 are called Fermat primes, and only five are known: 3, 5, 17, 257, and 65537. Fermat and Gauss knew of these. Nobody has been able to prove whether  there are any more. Thus, a regular -gon has a straightedge-and-compass construction if "n" is a product of distinct Fermat primes and any power of 2. The first few such are The RSA cryptosystem. Setting up an RSA system involves choosing large prime numbers and , computing and , and finding two numbers and such that . The numbers and (the "encryption key") are released to the public, and (the "decryption key") is kept private. A message, represented by an integer , where , is encrypted by computing . It is decrypted by computing . Euler's Theorem can be used to show that if  , then . The security of an RSA system would be compromised if the number could be efficiently factored or if could be efficiently computed without factoring . Unsolved problems. Lehmer's conjecture. If is prime, then . In 1932 D. H. Lehmer asked if there are any composite numbers such that divides . None are known. In 1933 he proved that if any such exists, it must be odd, square-free, and divisible by at least seven primes (i.e. ). In 1980 Cohen and Hagis proved that and that . Further, Hagis showed that if 3 divides then and . Carmichael's conjecture. This states that there  is no number with the property that for all other numbers , , . See Ford's theorem above. As stated in the main article, if there is a single counterexample to this conjecture, there must be infinitely many counterexamples, and the smallest one has at least ten billion digits in base 10. Riemann hypothesis. The Riemann hypothesis is true if and only if the inequality is true for all where is Euler's constant and is the product of the first primes. References. The "Disquisitiones Arithmeticae" has been translated from Latin into English and German. The German edition includes all of Gauss ' papers on number theory: all the proofs of quadratic reciprocity, the determination of the sign of the Gauss sum, the investigations into biquadratic reciprocity, and unpublished notes. References to the "Disquisitiones" are of the form Gauss, DA, art. "nnn".