The positive integer n is a pseudo-prime if an ≡ a (mod n), for all integers a. According to Fermat's Little Theorem, all primes are pseudo-primes. There are comparatively few pseudo-primes that are not primes; the first is 561. To determine whether an integer is prime or composite, it may be useful to test first whether or not it is a pseudo-prime. For most composite numbers, this will establish that they are composite.