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 A370218 #9 Feb 12 2024 08:39:08
%S A370218 1,3,15,93,639,4578,33423,246816,1838367,13788399,104011140,788315124,
%T A370218 5998380543,45794787678,350619595614,2691082393818,20699166876831,
%U A370218 159515321712480,1231354153215123,9519556856284218,73694926944160164,571201080979318470
%N A370218 Coefficient of x^n in the expansion of ( (1+x)^3 / (1-x^3)^3 )^n.
%F A370218 a(n) = Sum_{k=0..floor(n/3)} binomial(3*n+k-1,k) * binomial(3*n,n-3*k).
%F A370218 The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x / (1+x)^3 * (1-x^3)^3 ). See A369403.
%o A370218 (PARI) a(n, s=3, t=3, u=3) = sum(k=0, n\s, binomial(t*n+k-1, k)*binomial(u*n, n-s*k));
%Y A370218 Cf. A369403.
%K A370218 nonn
%O A370218 0,2
%A A370218 _Seiichi Manyama_, Feb 12 2024