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%I #13 Sep 19 2021 22:02:19
%S 3,5,17,21,27,33,35,37,39,41,45,49,55,57,59,61,65,69,71,75,87,89,93,
%T 95,101,107,109,115,119,125,129,131,137,139,141,145,149,151,153,155,
%U 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 Odd numbers k such that sigma(k^2) has an odd number of prime factors when counted with multiplicity.
%C Equally, odd numbers k such that A003415(sigma(k^2)) is odd, i.e., k^2 is in A347877. See A235991.
%C 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 Select[Range[1, 300, 2], OddQ[PrimeOmega[DivisorSigma[1, #^2]]] &] (* _Amiram Eldar_, Sep 19 2021 *)
%o (PARI) isA347885(n) = ((n%2)&&(bigomega(sigma(n^2))%2));
%Y Cf. A000203, A001222, A003415, A235991, A347870, A347877, A347882, A347886 (complement among A005408), A347887 (subsequence).
%K nonn
%O 1,1
%A _Antti Karttunen_, Sep 19 2021
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