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 A364020 #21 Jul 03 2023 00:59:42
%S A364020 0,1,0,2,0,3,-1,4,0,5,0,7,0,5,0,8,-1,9,0,10,-3,12,0,14,0,13,-1,10,0,
%T A364020 15,0,17,0,15,-5,21,-1,19,0,20,0,16,0,24,0,23,-1,28,-7,25,-3,27,0,25,
%U A364020 0,20,-1,29,0,35,0,32,-9,34,0,36,-1,30,0,25,0,43,0,35,0,38,-12,39,0,40,-3,42,0,39,-5,43,-1
%N A364020 Expansion of Sum_{k>0} k * x^(2*k) / (1 + x^(5*k)).
%H A364020 Seiichi Manyama, <a href="/A364020/b364020.txt">Table of n, a(n) for n = 1..10000</a>
%F A364020 G.f.: Sum_{k>0} (-1)^(k-1) * x^(5*k-3) / (1 - x^(5*k-3))^2.
%F A364020 a(n) = Sum_{d|n, n/d==2 (mod 5)} (-1)^(n/d) * d.
%t A364020 a[n_] := DivisorSum[n, (-1)^(n/#) * # &, Mod[n/#, 5] == 2 &]; Array[a, 100] (* _Amiram Eldar_, Jul 03 2023 *)
%o A364020 (PARI) a(n) = sumdiv(n, d, (n/d%5==2)*(-1)^(n/d)*d);
%Y A364020 Cf. A364019, A364021, A364022.
%Y A364020 Cf. A363898.
%K A364020 sign
%O A364020 1,4
%A A364020 _Seiichi Manyama_, Jul 01 2023