Types of well-founded trees and the homomorphism order
We introduce an ordinal-valued invariant of well-founded trees, called their type. We use type to give an alternative proof that the class of all well-founded trees is well-quasi-ordered by the homomorphism relation, and we establish a relationship between type and the usual ordinal rank of a well-founded tree, showing in particular that type provides a strictly coarser structural classification.
math.LO↗