OFFSET
4,1
LINKS
Andrew Howroyd, Table of n, a(n) for n = 4..50
EXAMPLE
For n=4, take 4 nodes labeled a,b,c,d. We can choose the root in 4 ways, say a, and it must be joined to b,c,d. Each of the three edges bc, bd, cd may or may not exist, so there are 4*8 = 32 = a(4) possibilities.
PROG
(PARI) seq(n)={Vec(serlaplace(sum(k=1, n, k*binomial(k-1, 3)*2^binomial(k-1, 2)*x^k/k!)/sum(k=0, n, 2^binomial(k, 2)*x^k/k!) + O(x*x^n)))} \\ Andrew Howroyd, Nov 23 2020
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Christian G. Bower, Jan 04 1999; suggested by Vlady Ravelomanana
EXTENSIONS
Terms a(13) and beyond corrected by Andrew Howroyd, Nov 23 2020
STATUS
approved