CONSTRUCTING Δ_3~0 USING TOPOLOGICALLY RESTRICTIVE COUNTABLE DISJOINT UNIONS

摘要

In a zero-dimensional Polish space, the Borel sets are generated from the clopen sets by repeatedly applying the operations of countable disjoint union and complementation. Here we look at topologically restrictive versions of the general countable disjoint union of sets, and obtain "construction principles" for Δ_3~0, i.e., sets which are both F_(σδ) and g_(δσ).