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 A303397 #13 Apr 25 2018 03:00:21
%S A303397 1,-4,4,-4,20,-36,52,-116,244,-500,964,-1876,3876,-7780,15332,-30628,
%T A303397 61684,-123460,246036,-491988,985492,-1971284,3939556,-7878068,
%U A303397 15762692,-31527428,63041220,-126078916,252185044,-504375460,1008698036,-2017385268,4034873268
%N A303397 Expansion of Product_{k>=1} (1 - 2*x^k)/(1 + 2*x^k).
%F A303397 a(n) ~ c * (-2)^n, where c = QPochhammer[-1, -1/2]/QPochhammer[-1/2] = 0.93943604828296530723602398257349307281... - _Vaclav Kotesovec_, Apr 25 2018
%t A303397 nmax = 40; CoefficientList[Series[Product[(1 - 2*x^k)/(1 + 2*x^k), {k, 1, nmax}], {x, 0, nmax}], x] (* _Vaclav Kotesovec_, Apr 25 2018 *)
%o A303397 (PARI) N=66; x='x+O('x^N); Vec(prod(k=1, N, (1-2*x^k)/(1+2*x^k)))
%Y A303397 Expansion of Product_{k>=1} (1 - b*x^k)/(1 + b*x^k): A002448 (b=1), this sequence (b=2), A303398 (b=3).
%Y A303397 Cf. A261584, A303345.
%K A303397 sign
%O A303397 0,2
%A A303397 _Seiichi Manyama_, Apr 23 2018