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 A370248 #10 Feb 13 2024 07:44:54
%S A370248 1,1,3,16,67,276,1200,5293,23427,104425,468428,2110725,9546256,
%T A370248 43315546,197088195,898910916,4108495491,18812770011,86285313327,
%U A370248 396332663094,1822878714492,8394131895424,38696042930251,178561943852670,824720550229584,3812313399877776
%N A370248 Coefficient of x^n in the expansion of ( 1/(1-x) * (1+x^3)^2 )^n.
%F A370248 a(n) = Sum_{k=0..floor(n/3)} binomial(2*n,k) * binomial(2*n-3*k-1,n-3*k).
%F A370248 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)^2 ). See A369266.
%o A370248 (PARI) a(n, s=3, t=2, u=1) = sum(k=0, n\s, binomial(t*n, k)*binomial((u+1)*n-s*k-1, n-s*k));
%Y A370248 Cf. A369266, A370214.
%K A370248 nonn
%O A370248 0,3
%A A370248 _Seiichi Manyama_, Feb 13 2024