A note on an effective Polish topology and Silver's Dichotomy theorem

Abstract

We define a Polish topology inspired from the Gandy-Harrington topology and show how it can be used to prove Silver's dichotomy theorem while remaining in the Polish realm. In this topology, a $\Pi^1_1$ equivalence relation decomposes into a "sum" of a clopen relation and a meager one. We characterize it as the largest regular toplogy with a basis included in $\Sigma^1_1$.