OFFSET
4,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 4..500
EXAMPLE
a(4) = 1: 1243.
a(5) = 13: 13254, 13524, 13542, 14235, 21354, 23154, 23514, 23541, 24135, 31254, 34125, 41253, 51243.
MAPLE
b:= proc(u, o) option remember; `if`(u+o=0, 1, add(
b(u-j, o+j-1), j=1..u)+add(convert(series(
b(u+j-1, o-j)*x^(o-j), x, 3), polynom), j=1..o))
end:
a:= n-> coeff(b(n, 0), x, 2):
seq(a(n), n=4..25);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Nov 14 2015
STATUS
approved