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 A370184 #13 Feb 11 2024 09:16:07
%S A370184 1,3,15,87,539,3458,22659,150594,1011131,6841779,46577430,318654900,
%T A370184 2188931699,15087882943,104301302218,722840860787,5020500381131,
%U A370184 34937184351049,243539967641271,1700255814753027,11886457488148674,83200718154710607,583026777685802256
%N A370184 Coefficient of x^n in the expansion of ( (1+x)^2 * (1+x+x^3) )^n.
%F A370184 a(n) = Sum_{k=0..floor(n/3)} binomial(n,k) * binomial(3*n-k,n-3*k).
%F A370184 The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x / ((1+x)^2 * (1+x+x^3)) ). See A369482.
%o A370184 (PARI) a(n, s=3, t=1, u=2) = sum(k=0, n\s, binomial(t*n, k)*binomial((t+u)*n-k, n-s*k));
%Y A370184 Cf. A116411, A370183.
%Y A370184 Cf. A369482, A370196.
%K A370184 nonn
%O A370184 0,2
%A A370184 _Seiichi Manyama_, Feb 11 2024