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 A307903 #14 May 05 2019 09:24:09
%S A307903 1,1,4,36,448,6875,124956,2624293,62537728,1667191653,49158400000,
%T A307903 1588285928306,55796298391296,2117279603738494,86299754734693696,
%U A307903 3760031421065559375,174374733095888748544,8575617145497637681301,445758339115421869936896,24417549315693295193935516
%N A307903 Coefficient of x^n in (1 + n*x + n*x^3)^n.
%F A307903 a(n) = Sum_{k=0..floor(n/3)} n^(n-2*k) * binomial(n,3*k) * binomial(3*k,k).
%F A307903 a(n) ~ exp(3*n^(1/3)/2^(2/3)) * n^(n - 1/6) / (2^(2/3)*sqrt(3*Pi)) * (1 - 79/(36 * 2^(1/3) * n^(1/3))). - _Vaclav Kotesovec_, May 05 2019
%t A307903 Flatten[{1, Table[n^n * HypergeometricPFQ[{1/3 - n/3, 2/3 - n/3, -n/3}, {1/2, 1}, -27/(4*n^2)], {n, 1, 20}]}] (* _Vaclav Kotesovec_, May 05 2019 *)
%o A307903 (PARI) {a(n) = polcoef((1+n*x+n*x^3)^n, n)}
%o A307903 (PARI) {a(n) = sum(k=0, n\3, n^(n-2*k)*binomial(n,3*k)*binomial(3*k,k))}
%Y A307903 Cf. A092366, A116411, A307904, A307905.
%K A307903 nonn
%O A307903 0,3
%A A307903 _Seiichi Manyama_, May 05 2019