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 A347877 #16 Feb 25 2022 17:02:49 %S A347877 2,4,5,9,10,13,16,17,20,25,26,29,32,34,37,40,41,45,52,53,58,61,64,68, %T A347877 72,73,74,80,82,89,90,97,98,101,104,106,109,113,116,117,122,128,136, %U A347877 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 %N A347877 Numbers k for which A003415(sigma(k)) is odd. %C A347877 Numbers k such that A342925(k) == 1 (mod 2). %C A347877 Squares present in this sequence are terms of A347885 squared. (There are no even squares present, see A235991 for the explanation). %H A347877 Antti Karttunen, <a href="/A347877/b347877.txt">Table of n, a(n) for n = 1..20000</a> %F A347877 For all n >= 1, A349909(a(n)) = n. - _Antti Karttunen_, Feb 23 2022 %t A347877 ad[1] = 0; ad[n_] := n * Total@(Last[#]/First[#]& /@ FactorInteger[n]); Select[Range[234], OddQ[ad[DivisorSigma[1, #]]] &] (* _Amiram Eldar_, Sep 19 2021 *) %Y A347877 Cf. A000203, A003415, A023194 (subsequence), A235991, A342925, A347872, A347873, A347885, A347878 (complement). %Y A347877 Cf. A347870 (characteristic function), A349909 (its partial sums). %K A347877 nonn %O A347877 1,1 %A A347877 _Antti Karttunen_, Sep 19 2021