login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

A262479
Number of ordered pairs (p,q) of permutations of [n] with equal up-down signatures and p(1)=q(1)=2.
2
1, 2, 8, 68, 952, 19312, 533544, 19209584, 873274112, 48904686848, 3307732742656, 265835173240320, 25041294346380800, 2732759857218994176, 342041866136488863232, 48672372916773453011968, 7813786873387535522529280, 1405550552561695118460387328
OFFSET
2,2
LINKS
MAPLE
b:= proc(u, o, h) option remember; `if`(u+o=0, 1,
add(add(b(u-j, o+j-1, h+i-1), i=1..u+o-h), j=1..u)+
add(add(b(u+j-1, o-j, h-i), i=1..h), j=1..o))
end:
a:= n-> (k-> b(k-1, n-k, n-k))(2):
seq(a(n), n=2..20);
CROSSREFS
Column k=2 of A262372.
Sequence in context: A093990 A361213 A156448 * A372315 A192550 A157752
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Sep 30 2015
STATUS
approved