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 A136223 #2 Mar 30 2012 18:37:08
%S A136223 1,3,21,208,2637,40731,742620,15624420,372892266,9959561867,
%T A136223 294465305959,9551090908795,337297690543923,12886076807637021,
%U A136223 529624555043780909,23305654066781507361,1093356525580359412557
%N A136223 Column 2 of triangle A136220; also equals column 0 of U^3 = A136236 where U = A136228.
%C A136223 P = A136220 is a triangular matrix where column k of P^3 equals column 0 of P^(3k+3) such that column 0 of P^3 equals column 0 of P shift one place left.
%o A136223 (PARI) {a(n)=local(P=Mat(1),U,PShR);if(n==0,1,for(i=0,n+1, PShR=matrix(#P,#P, r,c, if(r>=c,if(r==c,1,if(c==1,0,P[r-1,c-1]))));U=P*PShR^2; U=matrix(#P+1, #P+1, r,c, if(r>=c, if(r<#P+1,U[r,c], if(c==1,(P^3)[ #P,1],(P^(3*c-1))[r-c+1,1])))); P=matrix(#U, #U, r,c, if(r>=c, if(r<#U,P[r,c], (U^c)[r-c+1,1]))));P[n+3,3])}
%Y A136223 Cf. A136220 (P), A136228 (U), A136236 (U^3); other columns of P: A136221, A136222, A136224.
%K A136223 nonn
%O A136223 0,2
%A A136223 _Paul D. Hanna_, Dec 25 2007