OFFSET
0,9
COMMENTS
a(n) is the number of ways of placing n labeled balls into indistinguishable boxes, where in each filled box 7 balls are seen at the top.
a(n) is also the number of forests of labeled rooted trees of height at most 1, with n labels, where each root contains 7 labels.
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..556 (terms 0..200 from Alois P. Heinz)
N. J. A. Sloane, Transforms
FORMULA
E.g.f.: exp(exp(x)*x^7/7!).
MAPLE
a:= proc(n) option remember; `if`(n=0, 1,
add(binomial(n-1, j-1) *binomial(j, 7) *a(n-j), j=1..n))
end:
seq(a(n), n=0..30);
MATHEMATICA
a[n_] := a[n] = If[n == 0, 1, Sum[Binomial[n - 1, j - 1] *Binomial[j, 7]* a[n - j], {j, 1, n}]];
Table[a[n], {n, 0, 30}] (* Jean-François Alcover, Jun 11 2018, after Alois P. Heinz *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Oct 10 2008
STATUS
approved