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 A121427 #6 Mar 30 2012 18:36:58
%S A121427 1,2,15,178,2820,55410,1294776,35003430,1073540871,36805249870,
%T A121427 1394346324624,57831360118800,2605921998840420,126757491839620950,
%U A121427 6619466939158637640,369368127676399990338,21932876159270004129285
%N A121427 Main diagonal of rectangular table A121426.
%C A121427 Also main diagonal of square array A136737.
%F A121427 a(n) = [A121412^(n+1)](n+1,1) for n>=0; i.e., (n+1)-th term of column 1 in matrix power A121412^(n+1).
%o A121427 (PARI) {a(n)=local(H=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]=(H^i)[i-1, j]); )); H=B); return((H^(n+1))[n+2, 2])}
%Y A121427 Cf. A121426, A121412.
%Y A121427 Cf. A136737; A121413, A121417, A121422; A136741.
%K A121427 nonn
%O A121427 0,2
%A A121427 _Paul D. Hanna_, Aug 26 2006
%E A121427 Edited by _N. J. A. Sloane_, Oct 30 2008 at the suggestion of R. J. Mathar