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 A368455 #10 Dec 25 2023 09:59:21
%S A368455 1,0,4,20,180,1940,25204,381780,6609460,128728340,2785737204,
%T A368455 66312995540,1722049711540,48445655125140,1467738434962804,
%U A368455 47643726641609300,1649648835729498420,60688474959603256340,2363983970307953910004,97199476595104080495060
%N A368455 Expansion of e.g.f. 3*exp(-x) / (4 - exp(3*x)).
%F A368455 a(n) = (-1)^n + Sum_{k=1..n} 3^(k-1) * binomial(n,k) * a(n-k).
%o A368455 (PARI) a_vector(n) = my(v=vector(n+1)); for(i=0, n, v[i+1]=(-1)^i+sum(j=1, i, 3^(j-1)*binomial(i, j)*v[i-j+1])); v;
%Y A368455 Cf. A368453, A368454.
%K A368455 nonn
%O A368455 0,3
%A A368455 _Seiichi Manyama_, Dec 24 2023