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

1,1

Comments

First differs from A319240 in having 256.

Links

Table of n, a(n) for n=1..63.

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[#]]]&]

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.

Sequence in context: A377880 A375962 A319240 * A325270 A051278 A328028

Adjacent sequences: A331048 A331049 A331050 * A331052 A331053 A331054

Keyword

nonn

Author

Gus Wiseman, Jan 10 2020

Status

approved