Fermat's little theorem states that if p is a prime number, then for any integer a, the number a p - a is an integer multiple of p. In the notation of modular arithmetic, this is expressed as

For example, if a = 2 and p = 7, 27 = 128, and 128 - 2 = 7 x 18 is an integer multiple of 7.
If a is not divisible by p, Fermat's little theorem is equivalent to the statement that a p - 1 - 1 is an integer multiple of p, or in symbols

For example, if a = 2 and p = 7 then 26 = 64 and 64 - 1 = 63 is a multiple of 7.
Fermat's little theorem is the basis for the Fermat primality test and is one of the fundamental results of elementary number theory. The theorem is named after Pierre de Fermat, who stated it in 1640. It is called the "little theorem" to distinguish it from Fermat's last theorem.


== History ==

Pierre de Fermat first stated the theorem in a letter dated October 18, 1640, to his friend and confidant Frenicle de Bessy as the following:

p divides a p-1 - 1 whenever p is prime and a is coprime to p.

Fermat did not prove his assertion, only stating:

Et cette proposition est generalement vraie en toutes progressions et en tous nombres premiers; de quoi je vous envoierois la demonstration, si je n'apprehendois d'etre trop long.

(And this proposition is generally true for all progressions and for all prime numbers; the proof of which I would send to you, if I were not afraid to be too long.)

Euler provided the first published proof in 1736 in a paper entitled "Theorematum Quorundam ad Numeros Primos Spectantium Demonstratio", but Leibniz had given virtually the same proof in an unpublished manuscript from sometime before 1683.
The term "Fermat's Little Theorem" was first used in 1913 in Zahlentheorie by Kurt Hensel:

Fur jede endliche Gruppe besteht nun ein Fundamentalsatz, welcher der kleine Fermatsche Satz genannt zu werden pflegt, weil ein ganz spezieller Teil desselben zuerst von Fermat bewiesen worden ist."

(There is a fundamental theorem holding in every finite group, usually called Fermat's little Theorem because Fermat was the first to have proved a very special part of it.)

An early use in English occurs in A.A. Albert, Modern Higher Algebra (1937), which refers to "the so-called "little" Fermat theorem" on page 206.


=== Further history ===

Some mathematicians independently made the related hypothesis (sometimes incorrectly called the Chinese Hypothesis) that p is a prime if and only if . Indeed, the "only if" part is true, and a special case of Fermat's little theorem. However, the "if" part of this hypothesis is false: for example, , but 341 = 11 x 31 is a pseudoprime. See below.


== Proofs ==

Fermat gave his theorem without a proof. The first one who gave a proof was Gottfried Leibniz in a manuscript without a date, stating that he knew a proof before 1683.


== Generalizations ==
A slight generalization of the theorem, which immediately follows from it, is: if p is prime and m and n are positive integers such that
  then for every integer a coprime with p, we have  .
This follows as m is of the form , so 
In this form, the theorem is used to justify the RSA public key encryption method.
Fermat's little theorem is generalized by Euler's theorem: for any modulus n and any integer a coprime to n, we have

where ph(n) denotes Euler's totient function (which counts the integers between 1 and n that are coprime to n). This is indeed a generalization, because if n = p is a prime number, then ph(p) = p - 1.
This can be further generalized to Carmichael's theorem, as well as to Lagrange's theorem in group theory.
Fermat's little theorem also has a generalization in finite fields.


== Pseudoprimes ==
If a and p are coprime numbers such that a p-1 - 1 is divisible by p, then p need not be prime. If it is not, then p is called a pseudoprime to base a. F. Sarrus in 1820 found 341 = 11 x 31 as one of the first pseudoprimes, to base 2.
A number p that is a pseudoprime to base a for every number a coprime to p is called a Carmichael number (e.g. 561). Alternately, any number p satisfying the equality

is either a prime or Carmichael number.


== Converse ==
The converse of Fermat's little theorem is not generally true, as it fails for Carmichael numbers. However, a slightly stronger form of the theorem is true, and is known as Lehmer's theorem. The theorem is as follows:
If there exists an a such that

and for all primes q dividing p - 1

then p is prime.
This theorem forms the basis for the Lucas-Lehmer test, an important primality test.


== See also ==
Fractions with prime denominators: numbers with behavior relating to Fermat's little theorem
RSA
p-derivation
Frobenius endomorphism
Table of congruences


== Notes ==


== References ==
Long, Calvin T. (1972), Elementary Introduction to Number Theory (2nd ed.), Lexington: D. C. Heath and Company, LCCN 77171950 
Pettofrezzo, Anthony J.; Byrkit, Donald R. (1970), Elements of Number Theory, Englewood Cliffs: Prentice Hall, LCCN 71081766 
Paulo Ribenboim (1995). The New Book of Prime Number Records (3rd ed.). New York: Springer-Verlag. ISBN 0-387-94457-5. pp. 22-25, 49.
Janos Bolyai and the pseudoprimes (in Hungarian)


== External links ==
Necklace Visualization
Fermat's Little Theorem at cut-the-knot
Euler Function and Theorem at cut-the-knot
Fermat's Little Theorem and Sophie's Proof
Text and translation of Fermat's letter to Frenicle
Hazewinkel, Michiel, ed. (2001), "Fermat's little theorem", Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4 
Weisstein, Eric W., "Fermat's Little Theorem", MathWorld.
Weisstein, Eric W., "Fermat's Little Theorem Converse", MathWorld.
Grime, Dr. James. "Liar Numbers - Numberphile" (VIDEO). Brady Haran. Retrieved 4 February 2014.