OFFSET
9,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 9..200
FORMULA
MAPLE
b:= proc(u, o, t, k) option remember; `if`(u+o=0, 1,
`if`(t<k-1, add(b(u+j-1, o-j, t+1, k), j=1..o), 0)+
add(b(u-j, o+j-1, 0, k), j=1..u))
end:
a:= n-> b(n, 0, 0, 9)-b(n, 0, 0, 8):
seq(a(n), n=9..30);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Oct 12 2013
STATUS
approved