OFFSET
4,1
LINKS
Alois P. Heinz, Table of n, a(n) for n = 4..500
EXAMPLE
a(4) = 3: 2341, 3412, 3421.
a(5) = 23: 13452, 14523, 14532, 23415, 23514, 23541, 24351, 25341, 32451, 34125, 34152, 34215, 35124, 35142, 35214, 35412, 35421, 42351, 43512, 43521, 52341, 53412, 53421.
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^u, x, 3), polynom), j=1..o))
end:
a:= n-> coeff(b(n, 0), x, 2):
seq(a(n), n=4..25);
MATHEMATICA
b[u_, o_] := b[u, o] = If[u+o == 0, 1, Sum[b[u-j, o+j-1], {j, 1, u}] + Sum[Series[b[u+j-1, o-j] x^u, {x, 0, 3}] // Normal, {j, 1, o}]];
a[n_] := Coefficient[b[n, 0], x, 2];
a /@ Range[4, 25] (* Jean-François Alcover, Sep 28 2020, after Maple *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Nov 14 2015
STATUS
approved