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 A371385 #8 Mar 21 2024 09:19:16
%S A371385 1,8,109,1808,33283,653696,13419460,284479136,6179728951,136842057800,
%T A371385 3077436307141,70095952722752,1613743723323028,37490308916974496,
%U A371385 877802418598193488,20693109628871653184,490732756789852308223,11699199238845493854872
%N A371385 Expansion of (1/x) * Series_Reversion( x * (1-3*x)^3 / (1-x) ).
%H A371385 <a href="/index/Res#revert">Index entries for reversions of series</a>
%F A371385 a(n) = (1/(n+1)) * Sum_{k=0..n} 2^k * binomial(3*n+k+2,k) * binomial(3*n+1,n-k).
%o A371385 (PARI) my(N=20, x='x+O('x^N)); Vec(serreverse(x*(1-3*x)^3/(1-x))/x)
%o A371385 (PARI) a(n) = sum(k=0, n, 2^k*binomial(3*n+k+2, k)*binomial(3*n+1, n-k))/(n+1);
%Y A371385 Cf. A107841, A371380.
%Y A371385 Cf. A371363.
%K A371385 nonn
%O A371385 0,2
%A A371385 _Seiichi Manyama_, Mar 20 2024