Euler's totient function

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In
number theory Number theory (or arithmetic or higher arithmetic in older usage) is a branch of pure mathematics devoted primarily to the study of the integers and arithmetic function, integer-valued functions. German mathematician Carl Friedrich Gauss (1777– ...

, Euler's totient function counts the positive integers up to a given integer that are
relatively prime In number theory, two integer An integer (from the Latin wikt:integer#Latin, ''integer'' meaning "whole") is colloquially defined as a number that can be written without a Fraction (mathematics), fractional component. For example, 21, 4, 0, ...
to . It is written using the Greek letter
phi Phi (; uppercase Φ, lowercase φ or ϕ; grc, ϕεῖ ''pheî'' ; Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the dialects of the Greek ...

as $\varphi\left(n\right)$ or $\phi\left(n\right)$, 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 In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...

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 :''Outside number theory, the term multiplicative function is usually used for completely multiplicative functions. This article discusses number theoretic multiplicative functions.'' In number theory, a multiplicative function is an arithmetic ...
, meaning that if two numbers and are relatively prime, then . This function gives the
order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the habit of achieving a ...
of the multiplicative group of integers modulo (the Multiplicative group of integers modulo n, group of unit (ring theory), units of the ring (algebra), ring $\Z/n\Z$). It is also used for defining the RSA (cryptosystem), RSA encryption system.

# History, terminology, and notation

Leonhard Euler introduced the function in 1763.Sandifer, p. 203 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, James Joseph Sylvester, 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 function, 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 number, prime factor in common with .

# Computing Euler's totient function

There are several formulas for computing .

## Euler's product formula

It states :$\varphi\left(n\right) =n \prod_ \left\left(1-\frac\right\right),$ where the product is over the distinct prime numbers dividing . (For notation, see Arithmetical function#Notation, Arithmetical function.) An equivalent formulation for $n = p_1^ p_2^ \cdots p_r^$, where $p_1, p_2,\ldots,p_r$ are the distinct primes dividing ''n'', is:$\varphi(n) = p_1^(p_11)\,p_2^(p_21)\cdots p_r^(p_r1).$The proof of these formulas 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 :$\varphi \left\left(p^k\right\right) = p^k-p^ = p^\left(p-1\right) = p^k \left\left( 1 - \tfrac \right\right).$ ''Proof'': Since is a prime number, the only possible values of are , and the only way to have is if is a multiple of , i.e. , and there are such multiples less 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 $n = p_1^ p_2^ \cdots p_r^,$ where are prime numbers and each . (The case corresponds to the empty product.) Repeatedly using the multiplicative property of and the formula for gives :$\begin \varphi\left(n\right)&=& \varphi\left(p_1^\right)\, \varphi\left(p_2^\right) \cdots\varphi\left(p_r^\right)\\\left[.1em\right] &=& p_1^ \left(p_1-1\right)\, p_2^ \left(p_2-1\right) \cdots p_r^\left(p_r-1\right)\\\left[.1em\right] &=& p_1^ \left\left(1- \frac \right\right) p_2^ \left\left(1- \frac \right\right) \cdots p_r^\left\left(1- \frac \right\right)\\\left[.1em\right] &=& p_1^ p_2^ \cdots p_r^ \left\left(1- \frac \right\right) \left\left(1- \frac \right\right) \cdots \left\left(1- \frac \right\right)\\\left[.1em\right] &=&n \left\left(1- \frac \right\right)\left\left(1- \frac \right\right) \cdots\left\left(1- \frac \right\right). \end$ 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 $\$, excluding the sets of integers divisible by the prime divisors.

### Example

:$\varphi\left(20\right)=\varphi\left(2^2 5\right)=20\,\left(1-\tfrac12\right)\,\left(1-\tfrac15\right) =20\cdot\tfrac12\cdot\tfrac45=8.$ 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:$\varphi(20) = \varphi(2^2 5^1)= 2^(21)\,5^(51) = 2\cdot 1\cdot 1\cdot 4 = 8.$

## Fourier transform

The totient is the discrete Fourier transform of the Greatest common divisor, gcd, evaluated at 1. Let :$\mathcal \\left[m\right] = \sum\limits_^n x_k \cdot e^$ where for . Then :$\varphi \left(n\right) = \mathcal \\left[1\right] = \sum\limits_^n \gcd\left(k,n\right) e^.$ The real part of this formula is :$\varphi \left(n\right)=\sum\limits_^n \gcd\left(k,n\right) \cos .$ For example, using $\cos\tfrac5 = \tfrac4$ and $\cos\tfrac5 = \tfrac4$:$\begin \varphi(10) &=& \gcd(1,10)\cos\tfrac + \gcd(2,10)\cos\tfrac + \gcd(3,10)\cos\tfrac+\cdots+\gcd(10,10)\cos\tfrac\\ &=& 1\cdot(\tfrac4) + 2\cdot(\tfrac4) + 1\cdot(-\tfrac4) + 2\cdot(-\tfrac4) + 5\cdot (-1) \\ && +\ 2\cdot(-\tfrac4) + 1\cdot(-\tfrac4) + 2\cdot(\tfrac4) + 1\cdot(\tfrac4) + 10 \cdot (1) \\ &=& 4 . \end$Unlike 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 :$\sum_\varphi\left(d\right)=n,$ where the sum is over all positive divisors of , can be proven in several ways. (See Arithmetical function#Notation, 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 Root of unity#Elementary facts, 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: :$\tfrac,\,\tfrac,\,\tfrac,\,\tfrac,\, \tfrac,\,\tfrac,\,\tfrac,\,\tfrac,\, \tfrac,\,\tfrac,\,\tfrac,\,\tfrac,\, \tfrac,\,\tfrac,\,\tfrac,\,\tfrac,\, \tfrac,\,\tfrac,\,\tfrac,\,\tfrac.$ Put them into lowest terms: :$\tfrac,\,\tfrac,\,\tfrac,\,\tfrac,\, \tfrac,\,\tfrac,\,\tfrac,\,\tfrac,\, \tfrac,\,\tfrac,\,\tfrac,\,\tfrac,\, \tfrac,\,\tfrac,\,\tfrac,\,\tfrac,\, \tfrac,\,\tfrac,\,\tfrac,\,\tfrac$ 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 :$\varphi\left(n\right) = \sum_ \mu\left\left( d \right\right) \cdot \frac = n\sum_ \frac,$ where is the Möbius function, the
multiplicative function :''Outside number theory, the term multiplicative function is usually used for completely multiplicative functions. This article discusses number theoretic multiplicative functions.'' In number theory, a multiplicative function is an arithmetic ...
defined by $\mu\left(p\right) = -1$ and $\mu\left(p^k\right) = 0$ for each prime and . This formula may also be derived from the product formula by multiplying out $\prod_ (1 - \frac)$ to get $\sum_ \frac.$ An example:$\begin \varphi(20) &= \mu(1)\cdot 20 + \mu(2)\cdot 10 +\mu(4)\cdot 5 +\mu(5)\cdot 4 + \mu(10)\cdot 2+\mu(20)\cdot 1\\[.5em] &= 1\cdot 20 - 1\cdot 10 + 0\cdot 5 - 1\cdot 4 + 1\cdot 2 + 0\cdot 1 = 8. \end$

# 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 $\varphi\left(n\right) \ge \sqrt$, which is rather loose: in fact, the Limit superior and limit inferior, lower limit of the graph is proportional to .

# Euler's theorem

This states that if and are
relatively prime In number theory, two integer An integer (from the Latin wikt:integer#Latin, ''integer'' meaning "whole") is colloquially defined as a number that can be written without a Fraction (mathematics), fractional component. For example, 21, 4, 0, ...
then :$a^ \equiv 1\mod n.$ The special case where is prime is known as Fermat's little theorem. This follows from Lagrange's theorem (group theory), Lagrange's theorem and the fact that is the
order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the habit of achieving a ...
of the multiplicative group of integers modulo . The RSA (algorithm), RSA cryptosystem is based on this theorem: it implies that the inverse function, 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 Integer factorization, difficulty to factor large numbers we have the guarantee that no one else knows the factorization.

# Other formulae

• $a\mid b \implies \varphi\left(a\right)\mid\varphi\left(b\right)$
• $n \mid \varphi\left(a^n-1\right) \quad \text a,n > 1$
• $\varphi\left(mn\right) = \varphi\left(m\right)\varphi\left(n\right)\cdot\frac \quad\textd = \operatorname\left(m,n\right)$

Note the special cases

*$\varphi\left(2m\right) = \begin 2\varphi\left(m\right) &\text m \text \\ \varphi\left(m\right) &\text m \text \end$ *$\varphi\left\left(n^m\right\right) = n^\varphi\left(n\right)$
• $\varphi\left(\operatorname\left(m,n\right)\right)\cdot\varphi\left(\operatorname\left(m,n\right)\right) = \varphi\left(m\right)\cdot\varphi\left(n\right)$

Compare this to the formula

*$\operatorname\left(m,n\right)\cdot \operatorname\left(m,n\right) = m \cdot n$ (See least common multiple.)
• is even for . Moreover, if has distinct odd prime factors,
• For any and such that there exists an such that .
• $\frac=\frac$

where is the radical of an integer, radical of (the product of all distinct primes dividing ).

• $\sum_ \frac = \frac$
• $\sum_\!\!k = \tfrac12 n\varphi\left(n\right) \quad \textn>1$
• $\sum_^n\varphi\left(k\right) = \tfrac12 \left\left(1+ \sum_^n \mu\left(k\right)\left\lfloor\frac\right\rfloor^2\right\right) =\frac3n^2+O\left\left(n\left(\log n\right)^\frac23\left(\log\log n\right)^\frac43\right\right)$ ( cited in)
• $\sum_^n\frac = \sum_^n\frac\left\lfloor\frac\right\rfloor=\frac6n+O\left\left(\left(\log n\right)^\frac23\left(\log\log n\right)^\frac43\right\right)$
• $\sum_^n\frac = \fracn-\frac2+O\left\left(\left(\log n\right)^\frac23\right\right)$
• $\sum_^n\frac = \frac\left\left(\log n+\gamma-\sum_\frac\right\right)+O\left\left(\fracn\right\right)$

(where is the Euler–Mascheroni constant).

• $\sum_\stackrel \!\!\!\! 1 = n \frac + O \left \left( 2^ \right \right)$

where is a positive integer and is the number of distinct prime factors of .

## Menon's identity

In 1965 P. Kesava Menon proved :$\sum_ \!\!\!\! \gcd\left(k-1,n\right)=\varphi\left(n\right)d\left(n\right),$ 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: :$\phi=-\sum_^\infty\frac\log\left\left(1-\frac\right\right)$ and :$\frac=-\sum_^\infty\frac\log\left\left(1-\frac\right\right).$ Subtracting them gives :$\sum_^\infty\frac\log\left\left(1-\frac\right\right)=1.$ Applying the exponential function to both sides of the preceding identity yields an infinite product formula for : :$e= \prod_^ \left\left(1-\frac\right\right)\!\!^\frac.$ 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: :$\sum_^\infty \frac=\frac.$ The Lambert series generating function is :$\sum_^ \frac= \frac$ 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 :$\lim\sup \frac= 1,$ but as ''n'' goes to infinity, for all :$\frac\rightarrow\infty.$ These two formulae can be proved by using little more than the formulae for and the divisor function, divisor sum function . In fact, during the proof of the second formula, the inequality :$\frac < \frac < 1,$ true for , is proved. We also have :$\lim\inf\frac\log\log n = e^.$ Here is Euler–Mascheroni constant, Euler's constant, , so and . Proving this does not quite require the prime number theorem. Since goes to infinity, this formula shows that :$\lim\inf\frac= 0.$ In fact, more is true. :$\varphi\left(n\right) > \frac \quad\text n>2$ and :$\varphi\left(n\right) < \frac \quad\text n.$ 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 haveSándor, Mitrinović & Crstici (2006) pp.24–25 :$\varphi\left(1\right)+\varphi\left(2\right)+\cdots+\varphi\left(n\right) = \frac+O\left\left(n\left(\log n\right)^\frac23\left(\log\log n\right)^\frac43\right\right) \quad\textn\rightarrow\infty,$ due to Arnold Walfisz, its proof exploiting estimates on exponential sums due to Ivan Matveevich Vinogradov, I. M. Vinogradov and N. M. Korobov (this is currently the best known estimate of this type). The Big O notation, "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 provedRibenboim, p.38Sándor, Mitrinović & Crstici (2006) p.16 :$\begin \lim\inf \frac&= 0 \quad\text \\\left[5px\right] \lim\sup \frac&= \infty. \end$ In 1954 Andrzej Schinzel, Schinzel and Wacław Sierpiński, Sierpiński strengthened this, proving that the set :$\left\$ is Dense set, dense in the positive real numbers. They also proved that the set :$\left\$ 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.Guy (2004) p.144 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,Sándor & Crstici (2004) p.230 and indeed every positive integer has a multiple which is an even nontotient. The number of totient numbers up to a given limit is :$\frace^$ for a constant . If counted accordingly to multiplicity, the number of totient numbers up to a given limit is :$\Big\vert\\Big\vert = \frac \cdot x + R\left(x\right)$ where the error term is of order at most for any positive .Sándor et al (2006) p.22 It is known that the multiplicity of exceeds infinitely often for any .Sándor et al (2006) p.21Guy (2004) p.145

## 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,Sándor & Crstici (2004) p.229 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 .Sándor & Crstici (2004) p.228

# Applications

## Cyclotomy

In the last section of the Disquisitiones Arithmeticae, ''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 :2, 3, 4, 5, 6, 8, 10, 12, 15, 16, 17, 20, 24, 30, 32, 34, 40,... .

## 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 factored or if could be 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 .Guy (2004) p.142

## Carmichael's conjecture

This states that there is no number with the property that for all other numbers , , . See #Ford's theorem, 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.

*Carmichael function *Duffin–Schaeffer conjecture *Fermat's little theorem#Generalizations, Generalizations of Fermat's little theorem *Highly composite number *Multiplicative group of integers modulo n, Multiplicative group of integers modulo *Ramanujan sum *Totient summatory function *Dedekind psi function

# 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''. *. See paragraph 24.3.2. * * Dickson, Leonard Eugene, "History Of The Theory Of Numbers", vol 1, chapter 5 "Euler's Function, Generalizations; Farey Series", Chelsea Publishing 1952 *. * * * * * * * * * * * *.