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 A166901 #2 Mar 30 2012 18:37:18
%S A166901 1,4,21,156,1540,19160,288813,5123608,104657520,2420186616,
%T A166901 62514944778,1784255891484,55767065855228,1894463658611680,
%U A166901 69504774168222109,2738952451360200312,115380142451625516088,5174227834995200591840
%N A166901 Column 1 of triangle A166900.
%C A166901 Triangle A166900 transforms rows into diagonals in the table of coefficients of successive iterations of x+x^2 (cf. A122888).
%o A166901 (PARI) {a(n)=local(F=x, M, N, P); M=matrix(n+3, n+3, r, c, F=x;for(i=1, r+c-2, F=subst(F, x, x+x^2+x*O(x^(n+3)))); polcoeff(F, c)); N=matrix(n+2, n+2, 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+2, n+2, r, c, M[r+1, c]); (P~*N~^-1)[n+2, 2]}
%Y A166901 Cf. A166900, A166902, A166903, A122888, A135080.
%K A166901 nonn
%O A166901 0,2
%A A166901 _Paul D. Hanna_, Nov 27 2009