A262479 - OEIS

%I #23 Oct 26 2018 21:46:40

%S 1,2,8,68,952,19312,533544,19209584,873274112,48904686848,

%T 3307732742656,265835173240320,25041294346380800,2732759857218994176,

%U 342041866136488863232,48672372916773453011968,7813786873387535522529280,1405550552561695118460387328

%N Number of ordered pairs (p,q) of permutations of [n] with equal up-down signatures and p(1)=q(1)=2.

%H Alois P. Heinz, <a href="/A262479/b262479.txt">Table of n, a(n) for n = 2..150</a>

%p b:= proc(u, o, h) option remember; `if`(u+o=0, 1,

%p       add(add(b(u-j, o+j-1, h+i-1), i=1..u+o-h), j=1..u)+

%p       add(add(b(u+j-1, o-j, h-i), i=1..h), j=1..o))

%p     end:

%p a:= n-> (k-> b(k-1, n-k, n-k))(2):

%p seq(a(n), n=2..20);

%Y Column k=2 of A262372.

%K nonn

%O 2,2

%A _Alois P. Heinz_, Sep 30 2015