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 A371661 #23 Dec 26 2024 03:02:06
%S A371661 1,4,64,1424,36800,1036160,30843648,954671360,30415326208,
%T A371661 990831196160,32853724512256,1105132250898432,37620337933582336,
%U A371661 1293586791397064704,44863864476704768000,1567543145774827241472,55125711913212153954304
%N A371661 G.f. satisfies A(x) = 1 + x * A(x)^3 * (1 + A(x))^2.
%H A371661 Seiichi Manyama, <a href="/A371661/b371661.txt">Table of n, a(n) for n = 0..633</a>
%F A371661 a(n) = (1/n) * Sum_{k=0..floor((n-1)/2)} 4^(n-k) * binomial(n,k) * binomial(4*n-k,n-1-2*k) for n > 0.
%F A371661 a(n) = 2^n * A371669(n). - _Seiichi Manyama_, Dec 26 2024
%o A371661 (PARI) a(n) = if(n==0, 1, sum(k=0, (n-1)\2, 4^(n-k)*binomial(n, k)*binomial(4*n-k, n-1-2*k))/n);
%Y A371661 Cf. A271469, A363311, A371660.
%Y A371661 Cf. A371669.
%K A371661 nonn
%O A371661 0,2
%A A371661 _Seiichi Manyama_, Apr 01 2024