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A229890
Number of 9 up, 9 down permutations of [n].
2
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
0,12
LINKS
EXAMPLE
a(9) = 1: 123456789.
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);
CROSSREFS
Column k=9 of A229892.
Sequence in context: A023035 A128936 A000582 * A243744 A145459 A290893
KEYWORD
nonn,eigen
AUTHOR
Alois P. Heinz, Oct 02 2013
STATUS
approved