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 A303398 #17 Jul 31 2020 16:42:16
%S A303398 1,-6,12,-24,102,-312,840,-2544,7788,-23406,69816,-208968,628536,
%T A303398 -1886712,5654784,-16961856,50900934,-152709936,458084244,-1374231912,
%U A303398 4122828408,-12368549040,37105252680,-111315549552,333947845416,-1001844169854,3005528872008
%N A303398 Expansion of Product_{k>=1} (1 - 3*x^k)/(1 + 3*x^k).
%H A303398 Robert Israel, <a href="/A303398/b303398.txt">Table of n, a(n) for n = 0..2094</a>
%F A303398 a(n) ~ c * (-3)^n, where c = QPochhammer[-1, -1/3]/QPochhammer[-1/3] = 1.1824106844873309732830080836112464096086... - _Vaclav Kotesovec_, Apr 25 2018
%p A303398 N:= 100: # for a(0)..a(N)
%p A303398 G:= mul((1-3*x^k)/(1+3*x^k),k=1..N):
%p A303398 S:= series(G,x,N+1):
%p A303398 seq(coeff(S,x,n),n=0..N); # _Robert Israel_, Jul 31 2020
%t A303398 nmax = 30; CoefficientList[Series[Product[(1 - 3*x^k)/(1 + 3*x^k), {k, 1, nmax}], {x, 0, nmax}], x] (* _Vaclav Kotesovec_, Apr 25 2018 *)
%o A303398 (PARI) N=66; x='x+O('x^N); Vec(prod(k=1, N, (1-3*x^k)/(1+3*x^k)))
%Y A303398 Expansion of Product_{k>=1} (1 - b*x^k)/(1 + b*x^k): A002448 (b=1), A303397 (b=2), this sequence (b=3), A303402 (b=4).
%Y A303398 Cf. A303390.
%K A303398 sign
%O A303398 0,2
%A A303398 _Seiichi Manyama_, Apr 23 2018