An important question is: Is an integer prime if and only if it satisﬁes the Perrin condition, n divides xIt's always gratifying to see one's work referenced, and the letter provides some nice context for my research (better than I did in the paper itself!)._{n}? This question was raised by R. Perrin in 1899. A counterexample, now known as aPerrin pseudoprime, was not discovered until 1982: the smallest one is 271441. This is quite remarkable compared to, say, Fermat pseudoprimes with base 2, for which 341 is the smallest example. Recent work by J. Grantham [3] shows that there are inﬁnitely many Perrin pseudoprimes.

## Tuesday, March 22, 2011

### Letter to the Editor

Stan Wagon wrote a letter to the editor in the April 2011 Mathematics Magazine about Perrin's sequence and Perrin pseudoprimes. You may not be able to access it on-line (I can't), but it reads in part:

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