A229887 Number of 6 up, 6 down permutations of [n].

1, 1, 1, 1, 1, 1, 1, 1, 7, 28, 84, 210, 462, 924, 11220, 74283, 354563, 1363571, 4482323, 13058891, 226630385, 2101302530, 13794660782, 71790720848, 314709809876, 1206605493016, 27221495651416, 324504194839891, 2711270620975411, 17790823624994551

Offset

0,9

Links

Alois P. Heinz, Table of n, a(n) for n = 0..200

Example

a(7) = 1: 1234567.
a(8) = 7: 12345687, 12345786, 12346785, 12356784, 12456783, 13456782, 23456781.
a(9) = 28: 123456987, 123457986, ..., 245678931, 345678921.

Maple

b:= proc(u, o, t) option remember; `if`(u+o=0, 1, add(`if`(t=6,
       b(o-j, u+j-1, 1), b(u+j-1, o-j, t+1)), j=1..o))
    end:
a:= n-> b(0, n, 0):
seq(a(n), n=0..35);

Crossrefs

Column k=6 of A229892.

Sequence in context: A290994 A049017 A019501 * A243741 A369809 A145456

Adjacent sequences: A229884 A229885 A229886 * A229888 A229889 A229890

Keyword

nonn,eigen

Author

Alois P. Heinz, Oct 02 2013

Status

approved