A347877 Numbers k for which A003415(sigma(k)) is odd.

2, 4, 5, 9, 10, 13, 16, 17, 20, 25, 26, 29, 32, 34, 37, 40, 41, 45, 52, 53, 58, 61, 64, 68, 72, 73, 74, 80, 82, 89, 90, 97, 98, 101, 104, 106, 109, 113, 116, 117, 122, 128, 136, 137, 146, 148, 149, 153, 157, 160, 162, 164, 173, 178, 180, 181, 193, 194, 196, 197, 200, 202, 208, 212, 218, 226, 229, 232, 233, 234

Offset

1,1

Comments

Numbers k such that A342925(k) == 1 (mod 2).

Squares present in this sequence are terms of A347885 squared. (There are no even squares present, see A235991 for the explanation).

Links

Antti Karttunen, Table of n, a(n) for n = 1..20000

Formula

For all n >= 1, A349909(a(n)) = n. - _Antti Karttunen_, Feb 23 2022

Mathematica

ad[1] = 0; ad[n_] := n * Total@(Last[#]/First[#]& /@ FactorInteger[n]); Select[Range[234], OddQ[ad[DivisorSigma[1, #]]] &] (* _Amiram Eldar_, Sep 19 2021 *)

Crossrefs

Cf. A000203, A003415, A023194 (subsequence), A235991, A342925, A347872, A347873, A347885, A347878 (complement).

Cf. A347870 (characteristic function), A349909 (its partial sums).

Sequence in context: A274693 A024618 A089221 * A167180 A091271 A063985

Adjacent sequences: A347874 A347875 A347876 * A347878 A347879 A347880

Keyword

nonn

Author

_Antti Karttunen_, Sep 19 2021

Status

approved