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 A370216 #9 Feb 12 2024 08:38:59
%S A370216 1,1,1,10,49,151,532,2353,9745,37675,150851,624603,2561476,10426625,
%T A370216 42800031,176797510,730069649,3016004001,12492387775,51845882845,
%U A370216 215363387699,895504027855,3728271696139,15538300424315,64812978200068,270565786871401,1130394586039421
%N A370216 Coefficient of x^n in the expansion of ( (1+x) / (1-x^3)^3 )^n.
%F A370216 a(n) = Sum_{k=0..floor(n/3)} binomial(3*n+k-1,k) * binomial(n,n-3*k).
%F A370216 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)^3 ). See A369401.
%o A370216 (PARI) a(n, s=3, t=3, u=1) = sum(k=0, n\s, binomial(t*n+k-1, k)*binomial(u*n, n-s*k));
%Y A370216 Cf. A369401.
%K A370216 nonn
%O A370216 0,4
%A A370216 _Seiichi Manyama_, Feb 12 2024