cp's OEIS Frontend

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

%I A229890 #9 Oct 04 2013 07:41:14
%S A229890 1,1,1,1,1,1,1,1,1,1,1,10,55,220,715,2002,5005,11440,24310,48620,
%T A229890 880022,8438222,56936672,303159572,1354883102,5281317614,18427861739,
%U A229890 58640820239,172577535989,4529938877492,62058701943644,590100171030140,4371834356234285
%N A229890 Number of 9 up, 9 down permutations of [n].
%H A229890 Alois P. Heinz, <a href="/A229890/b229890.txt">Table of n, a(n) for n = 0..200</a>
%e A229890 a(9) = 1: 123456789.
%p A229890 b:= proc(u, o, t) option remember; `if`(u+o=0, 1, add(`if`(t=9,
%p A229890        b(o-j, u+j-1, 1), b(u+j-1, o-j, t+1)), j=1..o))
%p A229890     end:
%p A229890 a:= n-> b(0, n, 0):
%p A229890 seq(a(n), n=0..35);
%Y A229890 Column k=9 of A229892.
%K A229890 nonn,eigen
%O A229890 0,12
%A A229890 _Alois P. Heinz_, Oct 02 2013