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 A370286 #6 Feb 14 2024 10:47:55
%S A370286 1,3,17,102,645,4193,27764,186231,1261213,8604759,59053167,407217396,
%T A370286 2819252544,19583729766,136426565999,952743556907,6667916884701,
%U A370286 46755146944959,328398159653117,2310073990369926,16271915501598595,114757849228310355
%N A370286 Coefficient of x^n in the expansion of ( (1+x)^3 + x^2 )^n.
%F A370286 a(n) = Sum_{k=0..floor(n/2)} binomial(n,k) * binomial(3*n-3*k,n-2*k).
%F A370286 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^2) ).
%o A370286 (PARI) a(n) = sum(k=0, n\2, binomial(n, k)*binomial(3*n-3*k, n-2*k));
%Y A370286 Cf. A002426, A006139.
%Y A370286 Cf. A188686, A370287.
%Y A370286 Cf. A065065.
%K A370286 nonn
%O A370286 0,2
%A A370286 _Seiichi Manyama_, Feb 14 2024