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 A370246 #9 Feb 13 2024 07:41:18
%S A370246 1,1,3,13,51,201,813,3333,13779,57361,240153,1010109,4264989,18066777,
%T A370246 76745763,326796213,1394494803,5961639969,25528971369,109482236013,
%U A370246 470145451401,2021360463849,8700225608583,37484437325157,161647475666301,697673760945201
%N A370246 Coefficient of x^n in the expansion of ( 1/(1-x) * (1+x^3) )^n.
%F A370246 a(n) = Sum_{k=0..floor(n/3)} binomial(n,k) * binomial(2*n-3*k-1,n-3*k).
%F A370246 The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x * (1-x) / (1+x^3) ). See A071969.
%o A370246 (PARI) a(n, s=3, t=1, u=1) = sum(k=0, n\s, binomial(t*n, k)*binomial((u+1)*n-s*k-1, n-s*k));
%Y A370246 Cf. A071969, A370212.
%K A370246 nonn
%O A370246 0,3
%A A370246 _Seiichi Manyama_, Feb 13 2024