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 A120917 #3 Mar 30 2012 18:36:58
%S A120917 1,4,32,212,1504,10848,79696,596160,4520000,34673940,268538048,
%T A120917 2096374656,16475970896,130234435648,1034568731408,8254368150320,
%U A120917 66112337392256,531345216883584,4283682906179728,34632004320564416
%N A120917 Central terms of triangle A120914 (cascadence of (1+2x)^2).
%o A120917 (PARI) {a(n)=local(A,F=1+4*x+4*x^2,d=2,G=x,H=1+x,S=ceil(log(n+1)/log(d+1))); for(i=0,n,G=x*subst(F,x,G+x*O(x^n)));for(i=0,S,H=subst(H,x,x*G^d+x*O(x^n))*G/x); A=(x*H-y*subst(H,x,x*y^d +x*O(x^n)))/(x*subst(F,x,y)-y); polcoeff(polcoeff(A,n,x),n,y)}
%Y A120917 Cf. A120914, A120915, A120916, A120918.
%K A120917 nonn
%O A120917 0,2
%A A120917 _Paul D. Hanna_, Jul 17 2006