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 A335022 #10 Jan 22 2025 15:41:17
%S A335022 0,0,0,-2,0,1,0,-6,3,3,0,-9,0,5,8,-14,0,4,0,-11,10,9,0,-29,5,11,12,
%T A335022 -13,0,5,0,-30,14,15,12,-30,0,17,16,-39,0,9,0,-17,32,21,0,-69,7,18,20,
%U A335022 -19,0,13,16,-49,22,27,0,-61,0,29,40,-62,18,17,0,-23,26,21,0,-98,0,35,48,-25
%N A335022 a(n) = Sum_{d|n, 1 < d < n} (-1)^(d + 1) * d.
%C A335022 Difference between the sum of the odd nontrivial divisors of n and the sum of the even nontrivial divisors of n.
%H A335022 Antti Karttunen, <a href="/A335022/b335022.txt">Table of n, a(n) for n = 1..20000</a>
%F A335022 G.f.: Sum_{k>=2} (-1)^(k + 1) * k * x^(2*k) / (1 - x^k).
%t A335022 Table[DivisorSum[n, (-1)^(# + 1) # &, 1 < # < n &], {n, 1, 76}]
%t A335022 nmax = 76; CoefficientList[Series[Sum[(-1)^(k + 1) k x^(2 k)/(1 - x^k), {k, 2, nmax}], {x, 0, nmax}], x] // Rest
%o A335022 (PARI) a(n) = sumdiv(n, d, if ((d>1) && (d<n), (-1)^(d + 1)*d)); \\ _Michel Marcus_, May 20 2020
%Y A335022 Cf. A002129, A048050, A058344, A335021.
%K A335022 sign
%O A335022 1,4
%A A335022 _Ilya Gutkovskiy_, May 19 2020