All prime numbers are of the form 6k +/- 1 (except 2 and 3) where k is a positive integer ( k > 0 ).
It is easy to verify!
6k + 1 - might be prime
6k + 2 = 2(3k+1) so 2 is a factor
6k + 3 = 3(2k+1) so 3 is a factor
6k + 4 = 2(2k+2) so 2 is a factor
6k + 5 = might be prime
6k + 5 can also be written as 6(k-1) + 5 = 6k - 1.
So, every prime is of the form 6k +/- 1
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