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