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 A371657 #37 Aug 20 2025 11:03:51
%S A371657 1,3,27,333,4752,73764,1209492,20610693,361403937,6478386561,
%T A371657 118181952369,2186908154748,40949739595242,774474351098031,
%U A371657 14772979729013247,283878381945510621,5490264493926636912,106786725176131118523,2087502569999563971843
%N A371657 G.f. satisfies A(x) = 1 + x * A(x)^2 * (1 + A(x) + A(x)^2).
%H A371657 Seiichi Manyama, <a href="/A371657/b371657.txt">Table of n, a(n) for n = 0..756</a>
%F A371657 a(n) = (1/n) * Sum_{k=0..floor((n-1)/2)} 3^(n-k) * binomial(n,k) * binomial(3*n-k,n-1-2*k) for n > 0.
%o A371657 (PARI) a(n) = if(n==0, 1, sum(k=0, (n-1)\2, 3^(n-k)*binomial(n, k)*binomial(3*n-k, n-1-2*k))/n);
%Y A371657 Cf. A107708, A156016.
%Y A371657 Cf. A219534, A219537, A371658.
%K A371657 nonn
%O A371657 0,2
%A A371657 _Seiichi Manyama_, Apr 01 2024