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In number theory, 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 a p ≡ a ( mod p ) . {\displaystyle a^{p}\equiv a{\pmod {p}}.}
Using fast algorithms for modular exponentiation and multiprecision multiplication, the running time of this algorithm is O(k log 2 n log log n) = Õ(k log 2 n), where k is the number of times we test a random a, and n is the value we want to test for primality; see Miller–Rabin primality test for details.
Some of the proofs of Fermat's little theorem given below depend on two simplifications.. The first is that we may assume that a is in the range 0 ≤ a ≤ p − 1.This is a simple consequence of the laws of modular arithmetic; we are simply saying that we may first reduce a modulo p.
Near the beginning of the 20th century, it was shown that a corollary of Fermat's little theorem could be used to test for primality. [8] This resulted in the Pocklington primality test. [9] However, as this test requires a partial factorization of n − 1 the running time was still quite slow in the worst case.
The false statement that all numbers that pass the Fermat primality test for base 2 are prime is called the Chinese hypothesis. The smallest base-2 Fermat pseudoprime is 341. It is not a prime, since it equals 11·31, but it satisfies Fermat's little theorem: 2 340 ≡ 1 (mod 341) and thus passes the Fermat primality test for the base 2.
by Fermat's little theorem, () (this property alone defines the weaker notion of probable prime to base a, on which the Fermat test is based); the only square roots of 1 modulo n are 1 and −1. Hence, by contraposition , if n is not a strong probable prime to base a , then n is definitely composite, and a is called a witness for the ...
An Euler probable prime to base a is an integer that is indicated prime by the somewhat stronger theorem that for any prime p, a (p−1)/2 equals () modulo p, where () is the Jacobi symbol. An Euler probable prime which is composite is called an Euler–Jacobi pseudoprime to base a. The smallest Euler-Jacobi pseudoprime to base 2 is 561.
It is true that if n is prime, then (this is a special case of Fermat's little theorem), however the converse (if then n is prime) is false, and therefore the hypothesis as a whole is false. The smallest counterexample is n = 341 = 11×31.