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 A370284 #7 Feb 14 2024 10:48:03
%S A370284 1,3,21,168,1425,12483,111594,1011636,9264753,85510590,794087151,
%T A370284 7410887718,69446624910,653019755430,6158495001960,58226492157048,
%U A370284 551725482707505,5238008159399163,49814314319342424,474467729545936650,4525387365179378775
%N A370284 Coefficient of x^n in the expansion of 1/( (1-x)^3 - x^3 )^n.
%F A370284 a(n) = Sum_{k=0..floor(n/3)} binomial(n+k-1,k) * binomial(4*n-1,n-3*k).
%F A370284 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) ). See A369114.
%o A370284 (PARI) a(n) = sum(k=0, n\3, binomial(n+k-1, k)*binomial(4*n-1, n-3*k));
%Y A370284 Cf. A369114.
%K A370284 nonn
%O A370284 0,2
%A A370284 _Seiichi Manyama_, Feb 13 2024