The Number of Topological Types of Trees

Two graphs are of the same topological type if they can be mutually embedded into each other topologically. We show that there are exactly ℵ₁ distinct topological types of countable trees. In general, for any infinite cardinal κ there are exactly κ⁺ distinct topological types of trees of size κ. This solves a problem of van der Holst from 2005.