8621

This number is a composite.

This is one of only 2 four-digit composite numbers which, when used as "K" in Leonhard Euler's formula for generating many primes (X^2)+X+K, in which Euler originally used K=41, yields more primes than composites for the first 1,000 integer values of X. Note that the only other 4-digit composite number to yield 50+% primes in Euler's formula is 6887. [Morse]