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 A369399 #15 Feb 16 2024 09:54:05
%S A369399 1,1,1,3,11,31,86,277,937,3095,10275,35091,121662,423286,1481648,
%T A369399 5232315,18601843,66436069,238327939,858805613,3106856141,11277393837,
%U A369399 41062303214,149948280259,549027748390,2015108865850,7412690394406,27324968423054
%N A369399 Expansion of (1/x) * Series_Reversion( x / (1+x) * (1-x^3)^2 ).
%H A369399 <a href="/index/Res#revert">Index entries for reversions of series</a>
%F A369399 a(n) = (1/(n+1)) * Sum_{k=0..floor(n/3)} binomial(2*n+k+1,k) * binomial(n+1,n-3*k).
%F A369399 a(n) = (1/(n+1)) * [x^n] ( (1+x) / (1-x^3)^2 )^(n+1). - _Seiichi Manyama_, Feb 16 2024
%o A369399 (PARI) my(N=30, x='x+O('x^N)); Vec(serreverse(x/(1+x)*(1-x^3)^2)/x)
%o A369399 (PARI) a(n, s=3, t=2, u=1) = sum(k=0, n\s, binomial(t*(n+1)+k-1, k)*binomial(u*(n+1), n-s*k))/(n+1);
%Y A369399 Cf. A215340, A369401.
%Y A369399 Cf. A369296, A370214.
%K A369399 nonn
%O A369399 0,4
%A A369399 _Seiichi Manyama_, Jan 22 2024