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 A121423 #3 Mar 30 2012 18:36:58
%S A121423 1,3,18,169,2190,36360,737051,17645187,487025244,15219471545,
%T A121423 530951735025,20447695079658,861389893507518,39394187817328680,
%U A121423 1943446826192453505,102863050524539640435,5813722327999905078450
%N A121423 Column 2 of triangle A121420.
%C A121423 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 A121423 (PARI) {a(n)=local(A=Mat(1), B); for(m=1, n+3, 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+3, 3])}
%Y A121423 Cf. A121420 (triangle); other columns: A121421, A121422.
%K A121423 nonn
%O A121423 0,2
%A A121423 _Paul D. Hanna_, Aug 23 2006