A229890 Number of 9 up, 9 down permutations of [n].

1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 10, 55, 220, 715, 2002, 5005, 11440, 24310, 48620, 880022, 8438222, 56936672, 303159572, 1354883102, 5281317614, 18427861739, 58640820239, 172577535989, 4529938877492, 62058701943644, 590100171030140, 4371834356234285

Offset: 0

Author

Alois P. Heinz, Oct 02 2013

Keywords

• nonn
• eigen

Examples

			a(9) = 1: 123456789.
		

Links

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

Crossrefs

Column k=9 of A229892.

Programs

Maple

b:= proc(u, o, t) option remember; `if`(u+o=0, 1, add(`if`(t=9,
       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);