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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.

91 = 7 * 13 and (91) = (7) * (13). [Firoozbakht]

91 is the smallest pseudoprime satisfying the congruence 3^n = 3modn.