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 A166904 #2 Mar 30 2012 18:37:18
%S A166904 1,2,7,40,321,3361,43667,679806,12358885,257281501,6039232167,
%T A166904 157879127902,4550258562799,143367509714352,4903128661348411,
%U A166904 180907738215049666,7163333648262397913,303006716530386750233
%N A166904 Row sums of triangle A166900.
%C A166904 Triangle A166900 transforms rows into diagonals in the table of coefficients of successive iterations of x+x^2 (cf. A122888).
%o A166904 (PARI) {a(n)=local(F=x, M, N, P); M=matrix(n+2, n+2, r, c, F=x;for(i=1, r+c-2, F=subst(F, x, x+x^2+x*O(x^(n+2)))); polcoeff(F, c)); N=matrix(n+1, n+1, r, c, F=x;for(i=1, r, F=subst(F, x, x+x^2+x*O(x^(n+3)))); polcoeff(F, c)); P=matrix(n+1, n+1, r, c, M[r+1, c]); M=(P~*N~^-1); sum(k=1,n+1,M[n+1,k])}
%Y A166904 Cf. A166900, A166901, A166902, A166903, A122888, A135080.
%K A166904 nonn
%O A166904 0,2
%A A166904 _Paul D. Hanna_, Nov 27 2009