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 A371660 #18 Dec 25 2024 05:33:51
%S A371660 1,3,36,603,11745,249372,5599044,130735620,3142426428,77238209502,
%T A371660 1932396279066,49047725266101,1259884849971465,32690034127387431,
%U A371660 855528520866461010,22556952666651901761,598607836414445357145,15976563963437863357146
%N A371660 G.f. satisfies A(x) = 1 + x * A(x)^3 * (1 + A(x) + A(x)^2).
%H A371660 Seiichi Manyama, <a href="/A371660/b371660.txt">Table of n, a(n) for n = 0..685</a>
%F A371660 a(n) = (1/n) * Sum_{k=0..floor((n-1)/2)} 3^(n-k) * binomial(n,k) * binomial(4*n-k,n-1-2*k) for n > 0.
%o A371660 (PARI) a(n) = if(n==0, 1, sum(k=0, (n-1)\2, 3^(n-k)*binomial(n, k)*binomial(4*n-k, n-1-2*k))/n);
%Y A371660 Cf. A271469, A363311, A371661.
%K A371660 nonn
%O A371660 0,2
%A A371660 _Seiichi Manyama_, Apr 01 2024