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 A229887 #11 Oct 04 2013 07:04:50 %S A229887 1,1,1,1,1,1,1,1,7,28,84,210,462,924,11220,74283,354563,1363571, %T A229887 4482323,13058891,226630385,2101302530,13794660782,71790720848, %U A229887 314709809876,1206605493016,27221495651416,324504194839891,2711270620975411,17790823624994551 %N A229887 Number of 6 up, 6 down permutations of [n]. %H A229887 Alois P. Heinz, <a href="/A229887/b229887.txt">Table of n, a(n) for n = 0..200</a> %e A229887 a(7) = 1: 1234567. %e A229887 a(8) = 7: 12345687, 12345786, 12346785, 12356784, 12456783, 13456782, 23456781. %e A229887 a(9) = 28: 123456987, 123457986, ..., 245678931, 345678921. %p A229887 b:= proc(u, o, t) option remember; `if`(u+o=0, 1, add(`if`(t=6, %p A229887 b(o-j, u+j-1, 1), b(u+j-1, o-j, t+1)), j=1..o)) %p A229887 end: %p A229887 a:= n-> b(0, n, 0): %p A229887 seq(a(n), n=0..35); %Y A229887 Column k=6 of A229892. %K A229887 nonn,eigen %O A229887 0,9 %A A229887 _Alois P. Heinz_, Oct 02 2013