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 A121417 #6 Mar 30 2012 18:36:58
%S A121417 1,2,9,69,769,11346,208914,4613976,118840164,3496297632,115638728395,
%T A121417 4246267163601,171369282105510,7538270885559264,358926669220446804,
%U A121417 18389706733665138450,1008742283718489346668,58981158542987625464424
%N A121417 Column 1 of triangle A121416.
%C A121417 Also column 1 of square array A136737.
%C A121417 A121416 is the matrix square of triangle A121412; row n of triangle T=A121412 equals row (n-1) of T^(n+1) with an appended '1'.
%o A121417 (PARI) {a(n)=local(A=Mat(1), B); for(m=1, n+2, B=matrix(m, m); for(i=1, m, for(j=1, i, if(j==i, B[i,j]=1, B[i, j]=(A^i)[i-1, j]); )); A=B); return((A^2)[n+2, 2])}
%Y A121417 Cf. A121416 (triangle); other columns: A121418, A121419.
%Y A121417 Cf. A136737; A121413, A121422; A121427, A136741.
%K A121417 nonn
%O A121417 0,2
%A A121417 _Paul D. Hanna_, Aug 22 2006
%E A121417 Edited by _N. J. A. Sloane_, Oct 30 2008 at the suggestion of R. J. Mathar