OFFSET
6,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 6..145
EXAMPLE
a(7) = 21: each graph has one 2-node tree and 5 1-node trees and C(7,2) = 21.
MAPLE
T:= proc(n, k) option remember; `if`(k<0 or k>n, 0,
`if`(n=0, 1, add(binomial(n-1, i)*T(n-1-i, k-1)*
`if`(i<2, 1, i!/2 +(i+1)^(i-1)), i=0..n-k)))
end:
a:= n-> T(n, 6):
seq(a(n), n=6..25);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Aug 26 2012
STATUS
approved