005

5 is a Fermat prime, a Mersenne prime exponent, as well as a Fibonacci number. 5 is the first congruent number, as well as the length of the hypotenuse of the smallest integer-sided right triangle, making part of the smallest Pythagorean triple (3, 4, 5). 5 is the first safe prime and the first good prime.11 forms the first pair of sexy primes with 5.5 is the second Fermat prime, of a total of five known Fermat primes. 5 is also the first of three known Wilson primes