5 is an odd number.
5 is the first odd number to be a non-Mersenne number, but
5 is a Fermat prime, equal to 22 + 1.
5 is one of the three divisors of 10: 1, 2 and 5 are the divisors of 10.
5 is a square pyramidal number, equal to \sum_{n=0}^2n^2, which is 22 + 12 = 5.