A229888 Number of 7 up, 7 down permutations of [n].

1, 1, 1, 1, 1, 1, 1, 1, 1, 8, 36, 120, 330, 792, 1716, 3432, 48477, 368797, 2002429, 8685469, 31964725, 103593205, 303129685, 6156709438, 66139517554, 498859498016, 2961422519550, 14715247475500, 63593705003400, 245316996896400, 6498850067259615

Offset

0,10

Links

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

Example

a(8) = 1: 12345678.
a(9) = 8: 123456798, 123456897, 123457896, 123467895, 123567894, 124567893, 134567892, 234567891.

Maple

b:= proc(u, o, t) option remember; `if`(u+o=0, 1, add(`if`(t=7,
       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=7 of A229892.

Sequence in context: A278971 A000580 A290995 * A243742 A145457 A145136

Adjacent sequences: A229885 A229886 A229887 * A229889 A229890 A229891

Keyword

nonn,eigen

Author

Alois P. Heinz, Oct 02 2013

Status

approved