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%I #30 Jun 24 2018 09:01:01
%S 1,2,3,4,5,6,8,9,10,12,15,16,17,18,20,24,25,27,30,32,33,34,36,40,45,
%T 48,50,51,54,60,64,65,66,68,72,75,80,81,85,90,96,99,100,102,108,120,
%U 125,128,129,130,132,135,136,144,150,153,160,162,165,170,180,192
%N Numbers that can be factored as a product of numbers of the form 2^k+1 (A000051).
%C If a(n) and a(m) are in the sequence, so is a(n)*a(m).
%e a(11) = 15 = 3*5 = (2^1 + 1)*(2^2 + 1).
%t up = 192; t = Complement[1+2^Range[0, Ceiling@Log2@up], {9}]; a = {}; ric[p_, w_] := Block[{q = p}, If[w == {}, AppendTo[a, p], While[q <= up, ric[q, Rest@w]; q *= w[[1]]]]]; ric[1, t]; Union[a] (* _Giovanni Resta_, Jun 14 2018 *)
%Y Cf. A000051, A143512, A143513.
%K nonn
%O 1,2
%A _Nicholas Stearns_, Jun 10 2018
%E More terms from _Giovanni Resta_, Jun 14 2018
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