521

This number is a prime.

If p is prime, then it divides the pth term of the Perrin sequence: 0, 2, 3, 2, 5, 5, 7, 10, 12, 17, ... (each term is the sum of the two terms preceding the term before it). Often, if n > 1 divides the nth term, then n is prime. The first of infinitely many exceptions to this rule is the square of 521. There are only 17 such composites less than 10^9.