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%I #15 Jul 09 2018 21:25:30
%S 3,5,7,11,13,19,23,47
%N Smallest nonempty set S containing prime divisors of 2k + 1 for each k in S.
%e 2 * 3 + 1 = 7, which is in the set.
%e 2 * 5 + 1 = 11, which is also in the set.
%e 2 * 7 + 1 = 15 = 3 * 5, both of which are in the set.
%e 2 * 11 + 1 = 23, which is in the set.
%e 2 * 13 + 1 = 27 = 3^3, and 3 is in the set.
%e 2 * 19 + 1 = 39 = 3 * 13, both of which are in the set.
%e 2 * 23 + 1 = 47, which is in the set.
%e 2 * 47 + 1 = 95 = 5 * 19, both of which are in the set.
%t setSeek[s_List] := FactorInteger[2 * # + 1][[All, 1]]& /@ s // Flatten // Union; FixedPoint[Union[#, setSeek[#]]&, setSeek[{1}]] (* _Jean-François Alcover_, Apr 30 2014 *)
%Y Cf. A020588.
%K nonn,fini,full
%O 1,1
%A _David W. Wilson_
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