# 521

This number is a prime.

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If *p*is prime, then it divides the

*p*th 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

*n*th 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.

Submitted: 2008-04-10 15:40:57; Last Modified: 2009-02-20 21:38:13.

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