On an identity for the cycle indices of rooted tree automorphism groups (Q870039)

Summary: This note deals with a formula due to G. Labelle for the summed cycle indices of all rooted trees, which resembles the well-known formula for the cycle index of the symmetric group in some way. An elementary proof is provided as well as some immediate corollaries and applications, in particular a new application to the enumeration of \(k\)-decomposable trees. A tree is called \(k\)-decomposable in this context if it has a spanning forest whose components are all of size \(k\).