 His book "Liber Abaci" (1202) was the first to describe trial division for testing primality, again using divisors only up to the square root.In 1640 Pierre de Fermat stated (without proof) Fermat's little theorem (later proved by Leibniz and Euler).Fermat also investigated the primality of the Fermat numbers
formula_18, and Marin Mersenne studied the Mersenne primes, prime numbers of the form formula_19 with formula_20 itself a prime.