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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.

 

  Submitted: 2001-10-26 19:34:13;   Last Modified: 2008-01-30 11:28:00.



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