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91 is pseudoprime to 35 bases less than 91. [Redman] 10^{n} + 91 {+ 0, + 2} are twin primes for n = 1, 2, 3 and 4. [Luhn] 91 is the smallest nontrivial 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 is the smallest pseudoprime satisfying the congruence 3^n = 3modn.
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