OFFSET
0,4
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..250
MAPLE
b:= proc(n, i, j) option remember; `if`(n=0, 1, add(
`if`(k=i or k=j, 0, (t-> binomial(n, k)*b(t,
`if`(k>t, 0, k), `if`(i>t, 0, i)))(n-k)), k=1..n))
end:
a:= n-> b(n, 0$2):
seq(a(n), n=0..30);
MATHEMATICA
b[n_, i_, j_] := b[n, i, j] = If[n == 0, 1, Sum[If[k == i || k == j, 0, Function[t, Binomial[n, k]*b[t, If[k > t, 0, k], If[i > t, 0, i]]][n - k]], {k, 1, n}]];
a[n_] := b[n, 0, 0];
Table[a[n], {n, 0, 30}] (* Jean-François Alcover, May 24 2018, translated from Maple *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Sep 06 2015
STATUS
approved