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 A370244 #9 Feb 13 2024 07:38:37
%S A370244 1,1,9,37,221,1176,6759,38368,222189,1290367,7551534,44367918,
%T A370244 261789647,1549582126,9198837384,54740021712,326445873389,
%U A370244 1950448508265,11673082484595,69965814023259,419923664517546,2523379461715576,15180084331541402,91411979525372616
%N A370244 Coefficient of x^n in the expansion of ( 1/(1-x) * (1+x^2)^3 )^n.
%F A370244 a(n) = Sum_{k=0..floor(n/2)} binomial(3*n,k) * binomial(2*n-2*k-1,n-2*k).
%F A370244 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^2)^3 ). See A369262.
%o A370244 (PARI) a(n, s=2, t=3, u=1) = sum(k=0, n\s, binomial(t*n, k)*binomial((u+1)*n-s*k-1, n-s*k));
%Y A370244 Cf. A240688, A370243.
%Y A370244 Cf. A369262.
%K A370244 nonn
%O A370244 0,3
%A A370244 _Seiichi Manyama_, Feb 13 2024