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 A372458 #7 May 02 2024 09:46:28
%S A372458 1,3,25,225,2129,20723,205471,2063890,20931585,213864939,2198044805,
%T A372458 22699471171,235354244255,2448409104820,25544033624414,
%U A372458 267158874185420,2800191197529633,29405702263792875,309320021637262225,3258658594126096867,34376186445159365709
%N A372458 Coefficient of x^n in the expansion of 1 / ( (1-x) * (1-x-x^2)^2 )^n.
%F A372458 a(n) = Sum_{k=0..floor(n/2)} binomial(2*n+k-1,k) * binomial(4*n-k-1,n-2*k).
%F A372458 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-x^2)^2 ). See A368965.
%o A372458 (PARI) a(n, s=2, t=2, u=1) = sum(k=0, n\s, binomial(t*n+k-1, k)*binomial((t+u+1)*n-(s-1)*k-1, n-s*k));
%Y A372458 Cf. A368965, A372233.
%K A372458 nonn
%O A372458 0,2
%A A372458 _Seiichi Manyama_, May 01 2024