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Ninety-one (91, XCI) is the natural number following ninety and preceding ninety-two. It is a triangular number, the sum of the squares of the first six integers, as well as a composite number, with its divisors being 7 and 13, and its factorization is:

It is the smallest pseudoprime satisfying the congruence 3n = 3 mod n.

Ninety-one is also:

This article is about the number. For the year AD 91, see 91.