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In number theory, Euler's totient or phi function, φ(n), is an arithmetic function that counts the totatives of n, that is, the positive integers less than or equal to n that are relatively prime to n. Thus, if n is a positive integer, then φ(n) is the number of integers k in the range 1 ≤ k ≤ n for which gcd(n, k) = 1.^{[1]}^{[2]} The totient function is a multiplicative function, meaning that if two numbers m and n are relatively prime (to each other), then φ(mn) = φ(m)φ(n).^{[3]}^{[4]}
For example let n = 9. Then gcd(9, 3) = gcd(9, 6) = 3 and gcd(9, 9) = 9. The other six numbers in the range 1 ≤ k ≤ 9, that is 1, 2, 4, 5, 7 and 8 are relatively prime to 9. Therefore, φ(9) = 6. As another example, φ(1) = 1 since gcd(1, 1) = 1.
The totient function is important mainly because it gives the order of the multiplicative group of integers modulo n (the group of units of the ring ). See Euler's theorem. The totient function also plays a key role in the definition of the RSA encryption system.
Leonhard Euler introduced the function in 1763.^{[5]}^{[6]}^{[7]} 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 π to denote it: he wrote πD for "the multitude of numbers less than D, and which have no common divisor with it".^{[8]} The standard notation^{[6]}^{[9]} φ(A) comes from Gauss' 1801 treatise Disquisitiones Arithmeticae^{[10]} (Gauss however doesn't use parentheses around the argument and writes φA). Thus, it is usually called Euler's phi function or simply the phi function.
In 1879 J. J. Sylvester coined the term totient for this function,^{[11]} so it is also referred to as the totient function, the Euler totient, or Euler's totient. Jordan's totient is a generalization of Euler's.
The cototient of n is defined as n – φ(n), i.e. the number of positive integers less than or equal to n that are divisible by at least one prime that also divides n.
There are several formulae for the totient.
It states
where the product is over the distinct prime numbers dividing n. (The notation is described in the article Arithmetical function.)
The proof of Euler's product formula depends on two important facts.
This means that if gcd(m, n) = 1, then φ(mn) = φ(m) φ(n). (Sketch of proof: let A, B, C be the sets of residue classes moduloandcoprime to m, n, mn respectively; then there is a bijection between A × B and C, by the Chinese remainder theorem.)
That is, if p is prime and k ≥ 1 then
Proof: Since p is a prime number the only possible values of gcd(p^{k}, m) are 1, p, p^{2}, ..., p^{k}, and the only way for gcd(p^{k}, m) to not equal 1 is for m to be a multiple of p. The multiples of p that are less than or equal to p^{k} are p, 2p, 3p, ..., p^{k − 1}p = p^{k}, and there are p^{k − 1} of them. Therefore the other p^{k} − p^{k − 1} numbers are all relatively prime to p^{k}.
Proof of the formula: The fundamental theorem of arithmetic states that if n > 1 there is a unique expression for n,
where p_{1} < p_{2} < ... < p_{r} are prime numbers and each k_{i} ≥ 1. (The case n = 1 corresponds to the empty product.)
Repeatedly using the multiplicative property of φ and the formula for φ(p^{k}) gives
This is Euler's product formula.
In words, this says that the distinct prime factors of 36 are 2 and 3; half of the thirtysix integers from 1 to 36 are divisible by 2, leaving eighteen; a third of those are divisible by 3, leaving twelve numbers that are coprime to 36. And indeed there are twelve positive integers that are coprime with 36 and lower than 36: 1, 5, 7, 11, 13, 17, 19, 23, 25, 29, 31, and 35.
The totient is the discrete Fourier transform of the gcd, evaluated at 1: (Schramm (2008))
The real part of this formula is
Note that unlike the other two formulae (the Euler product and the divisor sum) this one does not require knowing the factors of n. However, it does involve the calculation of the greatest common divisor of n and every positive integer less than n, which would suffice to provide the factorization anyway.
Euler's classical formula^{[12]}^{[13]}
where the sum is over all positive divisors d of n, can be proven in several ways. (see Arithmetical function for notational conventions.)
One way is to note that φ(d) is also equal to the number of possible generators of the cyclic group C_{d}; specifically, if C_{d} = <g>, then g^{k} is a generator for every k coprime to d. Since every element of C_{n} generates a cyclic subgroup, and all φ(d) subgroups of C_{d} ≤ C_{n} are generated by some element of C_{n}, the formula follows.^{[14]} In the article Root of unity Euler's formula is derived by using this argument in the special case of the multiplicative group of the nth roots of unity.
This formula can also be derived in a more concrete manner.^{[15]} Let n = 20 and consider the fractions between 0 and 1 with denominator 20:
Put them into lowest terms:
First note that all the divisors of 20 are denominators. And second, note that there are 20 fractions. Which fractions have 20 as denominator? The ones whose numerators are relatively prime to 20 By definition this is φ(20) fractions. Similarly, there are φ(10) = 4 fractions with denominator 10 φ(5) = 4 fractions with denominator 5 and so on. Since the same argument works for any number, not just 20, the formula is established.
Möbius inversion gives
where μ is the Möbius function.
This formula may also be derived from the product formula by multiplying out to get
For the Euler totient function can be calculated as a limit involving the Riemann zeta function:^{[citation needed]}
where
is the Riemann zeta function, is the Möbius function, is e (mathematical constant), and is a divisor.
The first 99 values (sequence A000010 in OEIS) are shown in the table and graph below:
+0  +1  +2  +3  +4  +5  +6  +7  +8  +9  

0+  1  1  2  2  4  2  6  4  6  
10+  4  10  4  12  6  8  8  16  6  18 
20+  8  12  10  22  8  20  12  18  12  28 
30+  8  30  16  20  16  24  12  36  18  24 
40+  16  40  12  42  20  24  22  46  16  42 
50+  20  32  24  52  18  40  24  36  28  58 
60+  16  60  30  36  32  48  20  66  32  44 
70+  24  70  24  72  36  40  36  60  24  78 
80+  32  54  40  82  24  64  42  56  40  88 
90+  24  72  44  60  46  72  32  96  42  60 
The top line in the graph, y = n − 1, is a true upper bound. It is attained whenever n is prime. There is no lower bound that is a straight line of positive slope; no matter how gentle the slope of a line is, there will eventually be points of the plot below the line. More precisely, the lower limit of the graph is proportional to n/log log n rather than being linear.^{[16]}
This states that if a and n are relatively prime then
The special case where n is prime is known as Fermat's little theorem.
This follows from Lagrange's theorem and the fact that φ(n) is the order of the multiplicative group of integers modulo n.
The RSA cryptosystem is based on this theorem: it implies that the inverse of the function is the function where is the multiplicative inverse of modulo . The difficulty of decoding without knowing the private key is thus the difficulty of computing which is the same as factoring
and
(here γ is the Euler constant).
where m > 1 is a positive integer and ω(m) is the number of distinct prime factors of m. (a, b) is a standard abbreviation for gcd(a, b).^{[21]}
In 1965 P. Kesava Menon proved
where d(n) = σ_{0}(n) is the number of divisors of n.
Schneider^{[22]} 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 Euler's constant e
The proof is based on the formulae
The Dirichlet series for φ(n) may be written in terms of the Riemann zeta function as:^{[23]}
The Lambert series generating function is^{[24]}
which converges for q < 1.
Both of these are proved by elementary series manipulations and the formulae for φ(n).
In the words of Hardy & Wright, φ(n) is "always ‘nearly n’."^{[25]}
First^{[26]}
but as n goes to infinity,^{[27]} for all δ > 0
These two formulae can be proved by using little more than the formulae for φ(n) and the divisor sum function σ(n).
In fact, during the proof of the second formula, the inequality
true for n > 1, is proven.
We also have^{[16]}
Here γ is Euler's constant, γ = 0.577215665..., e^{γ} = 1.7810724..., e^{−γ} = 0.56145948... .
Proving this doesn't quite require the prime number theorem.^{[28]}^{[29]} Since log log (n) goes to infinity, this formula shows that
In fact, more is true.^{[30]}^{[31]}
Concerning the second inequality, 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."^{[31]}
For the average order, we have^{[18]}^{[32]}
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 O" stands for a quantity that is bounded by a constant times the function of "n" inside the parentheses (which is small compared to n^{2}).
This result can be used to prove^{[33]} that the probability of two randomly chosen numbers being relatively prime is
In 1950 Somayajulu proved^{[34]}^{[35]}
In 1954 Schinzel and Sierpiński strengthened this, proving^{[34]}^{[35]} that the set
is dense in the positive real numbers. They also proved^{[34]} that the set
is dense in the interval (0, 1).
A totient number is a value of Euler's totient function: that is, an m for which there is at least one x for which φ(x) = m. The valency or multiplicity of a totient number m is the number of solutions to this equation.^{[36]} A nontotient is a natural number which is not a totient number: there are infinitely many nontotients,^{[37]} and indeed every odd number has a multiple which is a nontotient.^{[38]}
The number of totient numbers up to a given limit x is
for a constant C = 0.8178146... .^{[39]}
If counted accordingly to multiplicity, the number of totient numbers up to a given limit x is
where the error term R is of order at most for any positive k.^{[40]}
It is known that the multiplicity of m exceeds m^{δ} infinitely often for any δ < 0.55655.^{[41]}^{[42]}
Ford (1999) proved that for every integer k ≥ 2 there is a totient number m of multiplicity k: that is, for which the equation φ(x) = m has exactly k solutions; this result had previously been conjectured by Wacław Sierpiński,^{[43]} and it had been obtained as a consequence of Schinzel's hypothesis H.^{[39]} Indeed, each multiplicity that occurs, does so infinitely often.^{[39]}^{[42]}
However, no number m is known with multiplicity k = 1. Carmichael's totient function conjecture is the statement that there is no such m.^{[44]}
In the last section of the Disquisitiones^{[45]}^{[46]} Gauss proves^{[47]} that a regular ngon can be constructed with straightedge and compass if φ(n) is a power of 2. If n is a power of an odd prime number the formula for the totient says its totient can be a power of two only if a) n is a first power and b) n − 1 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 ngon has a straightedgeandcompass construction if n is a product of distinct Fermat primes and any power of 2. The first few such n are^{[48]} 2, 3, 4, 5, 6, 8, 10, 12, 15, 16, 17, 20, 24, 30, 32, 34, 40, ... . (sequence A003401 in OEIS)
Setting up an RSA system involves choosing large prime numbers p and q, computing n = pq and k = φ(n), and finding two numbers e and d such that ed ≡ 1 (mod k). The numbers n and e (the "encryption key") are released to the public, and d (the "decryption key") is kept private.
A message, represented by an integer m, where 0 < m < n, is encrypted by computing S = m^{e} (mod n).
It is decrypted by computing t = S^{d} (mod n). Euler's Theorem can be used to show that if 0 < t < n, then t = m.
The security of an RSA system would be compromised if the number n could be factored or if φ(n) could be computed without factoring n.
If p is prime, then φ(p) = p − 1. In 1932 D. H. Lehmer asked if there are any composite numbers n such that φ(n)  n − 1. None are known.^{[49]}
In 1933 he proved that if any such n exists, it must be odd, squarefree, and divisible by at least seven primes (i.e. ω(n) ≥ 7). In 1980 Cohen and Hagis proved that n > 10^{20} and that ω(n) ≥ 14.^{[50]} Further, Hagis showed that if 3 divides n then n > 10^{1937042} and ω(n) ≥ 298848.^{[51]}^{[52]}
This states that there is no number n with the property that for all other numbers m, m ≠ n, φ(m) ≠ φ(n). 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.^{[36]}
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.
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