A331051 Numbers whose number of factorizations into factors > 1 (A001055) is even.

4, 6, 9, 10, 12, 14, 15, 18, 20, 21, 22, 25, 26, 28, 33, 34, 35, 38, 39, 44, 45, 46, 48, 49, 50, 51, 52, 55, 57, 58, 62, 63, 65, 68, 69, 72, 74, 75, 76, 77, 80, 82, 85, 86, 87, 91, 92, 93, 94, 95, 98, 99, 106, 108, 111, 112, 115, 116, 117, 118, 119, 121, 122

Offset: 1

Author

Gus Wiseman, Jan 10 2020

Keywords

• nonn

Comments

First differs from A319240 in having 256.

Crossrefs

Complement of A331050.

The version for powers of two (instead of evens) is A330977.

The version for primes (instead of evens) is A330991.

Cf. A001055, A001221, A001222, A002033, A005117, A045778, A045782, A045783, A330972, A330973.

Programs

Mathematica

facs[n_]:=If[n<=1,{{}},Join@@Table[Map[Prepend[#,d]&,Select[facs[n/d],Min@@#>=d&]],{d,Rest[Divisors[n]]}]];
Select[Range[300],EvenQ[Length[facs[#]]]&]