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 A370287 #8 Feb 14 2024 10:47:51
%S A370287 1,3,15,87,531,3333,21309,138015,902547,5946153,39406005,262404585,
%T A370287 1754316045,11767931451,79165530375,533883963567,3608242091091,
%U A370287 24432635451465,165721028062605,1125743155558677,7657535953619721,52151890274636463,355576809975214095
%N A370287 Coefficient of x^n in the expansion of ( (1+x)^3 + x^3 )^n.
%F A370287 a(n) = Sum_{k=0..floor(n/3)} binomial(n,k) * binomial(3*n-3*k,n-3*k).
%F A370287 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 + x^3) ).
%o A370287 (PARI) a(n) = sum(k=0, n\3, binomial(n, k)*binomial(3*n-3*k, n-3*k));
%Y A370287 Cf. A116411, A370285.
%Y A370287 Cf. A188686, A370286.
%Y A370287 Cf. A192132.
%K A370287 nonn
%O A370287 0,2
%A A370287 _Seiichi Manyama_, Feb 14 2024