Wadge hierarchy and Veblen hierarchy Part I: Borel sets of finite rank

Abstract

AbstractWe consider Borel sets of finite rank A ⊆ ∧ω where cardinality of Λ is less than some uncountable regular cardinal . We obtain a “normal form” of A, by finding a Borel set Ω, such that A and Ω continuously reduce to each other. In more technical terms: we define simple Borel operations which are homomorphic to ordinal sum, to multiplication by a countable ordinal, and to ordinal exponentiation of base , under the map which sends every Borel set A of finite rank to its Wadge degree.