Formally continuous functions on Baire space

摘要

A function from Baire space N~N to the natural numbers N is called formally continuous if it is induced by a morphism between the corresponding formal spaces. We compare formal continuity to two other notions of continuity on Baire space working in Bishop constructive mathematics: one is a function induced by a Brouweroperation (i.e., inductively defined neighbourhood function); the other is a function uniformly continuous near every compact image.We show that formal continuity is equivalent to the former while it is strictly stronger than the latter. The equivalence of formally continuous functions and those induced by Brouwer-operations requires Countable Choice.