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 A121421 #6 Mar 30 2012 18:36:58
%S A121421 1,3,12,76,711,9054,147471,2938176,69328365,1891371807,58575539361,
%T A121421 2030011517685,77827890696820,3270046577551695,149407542447596319,
%U A121421 7374639622066056408,391044078030333899385,22168014954558449549349
%N A121421 Column 0 of triangle A121420.
%C A121421 Also column 2 of square array A136733.
%C A121421 A121420 is the matrix cube of triangle A121412; row n of triangle T=A121412 equals row (n-1) of T^(n+1) with an appended '1'.
%o A121421 (PARI) {a(n)=local(A=Mat(1), B); for(m=1, n+1, 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^3)[n+1, 1])}
%Y A121421 Cf. A121420 (triangle); other columns: A121422, A121423.
%Y A121421 Cf. A136733; A101483, A121418; A121425.
%K A121421 nonn
%O A121421 0,2
%A A121421 _Paul D. Hanna_, Aug 23 2006
%E A121421 Edited by _N. J. A. Sloane_, Oct 30 2008 at the suggestion of R. J. Mathar