This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A347885 #13 Sep 19 2021 22:02:19 %S A347885 3,5,17,21,27,33,35,37,39,41,45,49,55,57,59,61,65,69,71,75,87,89,93, %T A347885 95,101,107,109,115,119,125,129,131,137,139,141,145,149,151,153,155, %U A347885 159,167,169,173,181,187,189,193,201,215,219,221,229,231,235,237,249,255,259,265,269,273,283,287,289,291,293,297,307 %N A347885 Odd numbers k such that sigma(k^2) has an odd number of prime factors when counted with multiplicity. %C A347885 Equally, odd numbers k such that A003415(sigma(k^2)) is odd, i.e., k^2 is in A347877. See A235991. %C A347885 A square root of any hypothetical odd square x present in A005820 (triperfect numbers) would be a member of this sequence, because bigomega(x) would be even, and bigomega(3*x) would be odd. See also A347887. %t A347885 Select[Range[1, 300, 2], OddQ[PrimeOmega[DivisorSigma[1, #^2]]] &] (* _Amiram Eldar_, Sep 19 2021 *) %o A347885 (PARI) isA347885(n) = ((n%2)&&(bigomega(sigma(n^2))%2)); %Y A347885 Cf. A000203, A001222, A003415, A235991, A347870, A347877, A347882, A347886 (complement among A005408), A347887 (subsequence). %K A347885 nonn %O A347885 1,1 %A A347885 _Antti Karttunen_, Sep 19 2021