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 A366203 #5 Oct 08 2023 09:24:59
%S A366203 1,2,12,156,3507,115692,5066364,276943568,18152243967,1387267590540,
%T A366203 121106707350928,11889022355301672,1296359140925188212,
%U A366203 155440199716271334648,20327081449263918542412,2879054747404226046119448,439060192463001381367975215,71727764882350305085962745740
%N A366203 a(n) = (1/n) * Sum_{k=0..n-1} binomial(n+k-1,k) * binomial(3*n,n-k-1) * (n-3)^k.
%C A366203 a(n) is the coefficient of x^n in expansion of series reversion of g.f. for n-gonal numbers (with signs).
%F A366203 a(n) = [x^n] Series_Reversion( x * (1 - (n - 3) * x) / (1 + x)^3 ).
%t A366203 Unprotect[Power]; 0^0 = 1; Table[1/n Sum[Binomial[n + k - 1, k] Binomial[3 n, n - k - 1] (n - 3)^k, {k, 0, n - 1}], {n, 1, 18}]
%t A366203 Table[Binomial[3 n, n - 1] Hypergeometric2F1[1 - n, n, 2 (n + 1), 3 - n]/n, {n, 1, 18}]
%t A366203 Table[SeriesCoefficient[InverseSeries[Series[x (1 - (n - 3) x)/(1 + x)^3, {x, 0, n}], x], {x, 0, n}], {n, 1, 18}]
%Y A366203 Cf. A001764, A060354, A113207, A179848, A263843, A323208, A365816, A365817, A365818, A366204.
%K A366203 nonn
%O A366203 1,2
%A A366203 _Ilya Gutkovskiy_, Oct 04 2023