Euler's theorem
In number theory, Euler's theorem (also known as the Fermat–Euler theorem or Euler's totient theorem) states that, if n and a are coprime positive integers, then is congruent to modulo n, where denotes Euler's totient function; that is
This article is about Euler's theorem in number theory. For other uses, see List of things named after Leonhard Euler § Theorems.
In 1736, Leonhard Euler published a proof of Fermat's little theorem[1] (stated by Fermat without proof), which is the restriction of Euler's theorem to the case where n is a prime number. Subsequently, Euler presented other proofs of the theorem, culminating with his paper of 1763, in which he proved a generalization to the case where n is not prime.[2]
The converse of Euler's theorem is also true: if the above congruence is true, then and must be coprime.
The theorem is further generalized by some of Carmichael's theorems.
The theorem may be used to easily reduce large powers modulo . For example, consider finding the ones place decimal digit of , i.e. . The integers 7 and 10 are coprime, and . So Euler's theorem yields , and we get .
In general, when reducing a power of modulo (where and are coprime), one needs to work modulo in the exponent of :
Euler's theorem underlies the RSA cryptosystem, which is widely used in Internet communications. In this cryptosystem, Euler's theorem is used with n being a product of two large prime numbers, and the security of the system is based on the difficulty of factoring such an integer.
The Disquisitiones Arithmeticae has been translated from Gauss's Ciceronian Latin into English and German. The German edition includes all of his 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reciprocity, the determination of the sign of the Gauss sum, the investigations into biquadratic reciprocity, and unpublished notes.