number of labellings on a tree with given degrees

Tags: #theorem

Statement

Let d1,…,dn with each di≥1 and ∑di=2(n−1). The number of labelled trees on n vertices with the degree of the ith vertex di is equal to

(n−2d1−1,d2−1,…,dn−1)=(n−2)!(d1−1)!(d2−1)!…(dn−1)!

Proof

This is exactly the coefficient of x1d1…xndn in Fn defined in proof 1 of Cayley's formula.

For proof 2, note that these correspond to codes (c1,…,cn−2) such that each i appears di−1 times in the code.