2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/139036We give, for some Borel sets of a product of two Polish spaces, including the Borel sets with countable sections, a Hurewicz-like characterization of those which cannot become a transfinite difference of open sets by changing the two Polish topologies.LogicGeneral Topology54H05text