The homeomorphism problem for countable topological spaces

Abstract

We consider the homeomorphism problem for countable topological spaces and investigate its descriptive complexity as an equivalence relation. It is shown that even for countable metric spaces the homeomorphism problem is strictly more complicated than the isomorphism problem for countable graphs and indeed it is not Borel reducible to any orbit equivalence relation induced by a Borel action of a Polish group. We also characterize the relative complexity of some other equivalence relations arising in the study.