A347886 - OEIS

%I #11 Sep 19 2021 22:02:26

%S 1,7,9,11,13,15,19,23,25,29,31,43,47,51,53,63,67,73,77,79,81,83,85,91,

%T 97,99,103,105,111,113,117,121,123,127,133,135,143,147,157,161,163,

%U 165,171,175,177,179,183,185,191,195,197,199,203,205,207,209,211,213,217,223,225,227,233,239,241,243,245,247,251,253

%N Odd numbers k such that sigma(k^2) has an even number of prime factors when counted with multiplicity.

%C Equally, odd numbers k such that A003415(sigma(k^2)) is even, i.e., k^2 is in A347878. See A235991.

%H <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>

%t Select[Range[1, 250, 2], EvenQ[PrimeOmega[DivisorSigma[1, #^2]]] &] (* _Amiram Eldar_, Sep 19 2021 *)

%o (PARI) isA347886(n) = ((n%2)&&!(bigomega(sigma(n^2))%2));

%Y Cf. A000203, A001222, A003415, A005820, A235991, A342923, A342925, A347391, A347870, A347878, A347882, A347885 (complement among A005408).

%K nonn

%O 1,2

%A _Antti Karttunen_, Sep 19 2021