OFFSET
0,2
COMMENTS
The expected number of trees in each such forest a(n)/A089140(offset) approaches 3 as n gets large.
FORMULA
a(n) = Sum_{k=0..n} binomial(n-1,k-1)*n^(n-k)*2^k*k = 6*n*(n+2)^(n-2).
MATHEMATICA
nn = 18; tx = Sum[n^(n - 1) x^n/n!, {n, 1, nn}]; Range[0, nn]! CoefficientList[Series[D[Exp[y tx]^2, y] /. y -> 1, {x, 0, nn}], x]
CROSSREFS
KEYWORD
nonn
AUTHOR
Geoffrey Critzer, May 10 2013
STATUS
approved