645

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Summary

Factorization

The number 645 is a square-free number with prime factors 3, 5, and 43:

$\!645=3*5*43$

Properties and families

Property or family	Parameter values	First few numbers satisfying the property	Proof of satisfaction/containment/membership
Poulet number (also called Sarrus number) (Fermat pseudoprime to base 2)	third Poulet number	341, 561, 645, 1105, 1387, 1729, 1905, 2047, [SHOW MORE] 2465, 2701, 2821, 3277, 4033, 4369, 4371, 4681, 5461, 6601, 7957, 8321, 8481, 8911, 10261, 10585, 11305, 12801, 13741, 13747, 13981, 14491, 15709, 15841, 16705, 18705, 18721, 19951, 23001, 23377, 25761, 29341 View list on OEIS	$2^{28}\equiv 1{\pmod {645}}$, and 28 divides 644, so $2^{644}\equiv 1{\pmod {645}}$.