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 A374080 #7 Jun 28 2024 14:01:08
%S A374080 1,0,0,-1,-1,0,0,0,-1,0,1,1,0,0,0,1,0,0,0,1,1,-1,-1,0,0,0,-1,0,1,0,-1,
%T A374080 -1,0,0,-1,-1,1,1,0,-1,0,1,0,-1,0,1,1,-1,-1,1,1,0,0,1,2,-1,-1,0,1,0,
%U A374080 -2,0,2,1,-1,-1,1,1,-1,-1,1,1,-1,-2,0,1,-1,-2,1,2,0,-2,-1,2,0,-2
%N A374080 Expansion of Product_{k>=1} (1 - x^(4*k-1)) * (1 - x^(4*k)).
%H A374080 Ilya Gutkovskiy, <a href="/A374080/a374080.jpg">Scatterplot of a(n) up to n=10000</a>
%t A374080 nmax = 85; CoefficientList[Series[Product[(1 - x^(4 k - 1)) (1 - x^(4 k)), {k, 1, nmax}], {x, 0, nmax}], x]
%t A374080 a[0] = 1; a[n_] := a[n] = -(1/n) Sum[DivisorSum[k, # &, Or[Mod[#, 4] == 0, Mod[#, 4] == 3] &] a[n - k], {k, 1, n}]; Table[a[n], {n, 0, 85}]
%Y A374080 Cf. A010815, A014601, A035364, A081362, A082303, A106459, A137569, A274719, A284316, A301505, A374058, A374060, A374081.
%K A374080 sign
%O A374080 0,55
%A A374080 _Ilya Gutkovskiy_, Jun 27 2024