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 A307521 #13 May 14 2021 02:56:09
%S A307521 1,2,2,2,0,2,-2,2,0,-4,8,-4,-26,112,-288,560,-832,782,274,-3378,9424,
%T A307521 -17498,21182,-2154,-78180,284594,-700018,1381802,-2250316,2877674,
%U A307521 -2172870,-1955998,12715122,-33812990,67322842,-108956110,139447006,-110023870,-83188990,651268018
%N A307521 Expansion of Product_{k>=1} ((1 + x)^k + x^k)/((1 + x)^k - x^k).
%F A307521 G.f.: 1/theta_4(x/(1 + x)), where theta_4() is the Jacobi theta function.
%t A307521 m = 39; CoefficientList[Series[Product[((1 + x)^k + x^k)/((1 + x)^k - x^k), {k, 1, m}], {x, 0, m}], x] (* _Amiram Eldar_, May 14 2021 *)
%o A307521 (PARI) N=66; x='x+O('x^N); Vec(prod(k=1, N, ((1+x)^k+x^k)/((1+x)^k-x^k)))
%Y A307521 Cf. A002448, A307520, A307522, A318570.
%K A307521 sign
%O A307521 0,2
%A A307521 _Seiichi Manyama_, Apr 12 2019