2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/139044Let xi be a non-null countable ordinal. We study the Borel subsets of the plane that can be made $\bormxi$ by refining the Polish topology on the real line. These sets are called potentially $\bormxi$. We give a Hurewicz-like test to recognize potentially $\bormxi$ sets.LogicGeneral Topology03E15, 54H05text