639

639 = 3 x 3 x 71.

639 is the sum of the first 20 primes.

639 is the smallest number that is the sum of the first several primes and is congruent with 3 modulo 4.

639 is a number n whose fifth root has a decimal part that begins with n.

639 has the representation 639 = 3⁶ - 90.

Source: Number Gossip