#math trivia #131 solution

#math trivia for #May10:  #131 is a prime, 13 is a prime, and 31 is a prime.  For what other day-numbers xyz are xyz, xy and yz all prime?

Answer:  This is a good pattern matching problem.  The shortest approach is probably to look at prime xy values and see which z’s work.  We’ll allow the case where x = 0; this is  convenient because xyz = yz, so only one additional prime test is needed if y is prime.

  • 02:  023, 029
  • 03:  031, 037
  • 05:  053, 059
  • 07:  071, 073, 079
  • 11:  113
  • 13:  131, 137
  • 17:  173, 179
  • 19:  197
  • 23:  237
  • 29:  none
  • 31:  311, 313, 317

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