A305759 Numbers that can be factored as a product of numbers of the form 2^k+1 (A000051).

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, 48, 50, 51, 54, 60, 64, 65, 66, 68, 72, 75, 80, 81, 85, 90, 96, 99, 100, 102, 108, 120, 125, 128, 129, 130, 132, 135, 136, 144, 150, 153, 160, 162, 165, 170, 180, 192

Offset: 1

Author

Nicholas Stearns, Jun 10 2018

Keywords

• nonn

Comments

If a(n) and a(m) are in the sequence, so is a(n)*a(m).

Examples

			a(11) = 15 = 3*5 = (2^1 + 1)*(2^2 + 1).
		

Crossrefs

Cf. A000051, A143512, A143513.

Programs

Mathematica

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 *)

Extensions

More terms from Giovanni Resta, Jun 14 2018