91

This number is a composite.

91 is pseudoprime to 35 bases less than 91. [Redman]

10ⁿ + 91 {+ 0, + 2} are twin primes for n = 1, 2, 3 and 4. [Luhn]

91 is the smallest non-trivial odd composite (that is, its prime factors [7, 13] are not, at first glance, obvious). Every smaller odd composite is either a familiar square, ends in 5, has a digit sum that is a multiple of 3, or is obviously divisible by 11.

Prime numbers that end with "91" occur more often than any other 2-digit ending among the first ten million primes. [Gaydos]

91 is a Fermat pseudoprime.