Reverse mathematics of compact countable second-countable spaces

Abstract

We study the reverse mathematics of the theory of countable second-countable topological spaces, with a focus on compactness. We show that the general theory of such spaces works as expected in the subsystem $\mathsf{ACA}_0$ of second-order arithmetic, but we find that many unexpected pathologies can occur in weaker subsystems. In particular, we show that $\mathsf{RCA}_0$ does not prove that every compact discrete countable second-countable space is finite and that $\mathsf{RCA}_0$ does not prove that the product of two compact countable second-countable spaces is compact. To circumvent these pathologies, we introduce strengthened forms of compactness, discreteness, and Hausdorffness which are better behaved in subsystems of second-order arithmetic weaker than $\mathsf{ACA}_0$.