Comments by "disgruntledtoons" (@disgruntledtoons) on "What do 5, 13 and 563 have in common?" video.

  1. If p is composite, then (p-1)! will be divisible by p (because the factorial will include one or more pairs of numbers whose product is p), and therefore (p-1)! + 1 cannot be divisible by p. The other half of the theorem is trickier.
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