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 A307905 #19 Mar 28 2023 08:00:03
%S A307905 1,1,4,30,304,3875,59631,1076383,22309120,522262245,13631508400,
%T A307905 392535959156,12362973152751,422774554883590,15600699362473876,
%U A307905 617888566413340503,26145122799198386944,1177107512023013681429,56185125998674634494980,2834081165961033246374350
%N A307905 Coefficient of x^n in (1 + n*x + x^3)^n.
%H A307905 Robert Israel, <a href="/A307905/b307905.txt">Table of n, a(n) for n = 0..385</a>
%F A307905 a(n) = Sum_{k=0..floor(n/3)} n^(n-3*k) * binomial(n,3*k) * binomial(3*k,k).
%F A307905 a(n) ~ c * n^n, where c = Sum_{k>=0} 1/(k!*(2*k)!) = HypergeometricPFQ[{}, {1/2, 1}, 1/4] = 1.52106585051363080966025715155941607334728986626976774617... - _Vaclav Kotesovec_, May 05 2019
%p A307905 f:= n -> coeff((1+n*x+x^3)^n,x,n):
%p A307905 map(f, [$0..30]); # _Robert Israel_, Mar 27 2023
%t A307905 Flatten[{1, Table[n^n * HypergeometricPFQ[{1/3 - n/3, 2/3 - n/3, -n/3}, {1/2, 1}, -27/(4*n^3)], {n, 1, 20}]}] (* _Vaclav Kotesovec_, May 05 2019 *)
%o A307905 (PARI) {a(n) = polcoef((1+n*x+x^3)^n, n)}
%o A307905 (PARI) {a(n) = sum(k=0, n\3, n^(n-3*k)*binomial(n,3*k)*binomial(3*k,k))}
%Y A307905 Cf. A116411, A186925, A307903, A307904.
%K A307905 nonn
%O A307905 0,3
%A A307905 _Seiichi Manyama_, May 05 2019