On the Persistent Difficulty of Disjunction
We want to show, in this paper, that, in intuitionistic analysis, the union of two closed subsets of Baire space N is not always closed, and that, more generally, the union of a closed set and a II n 0 -set is not always II n+1 0 . In the proof of this fact we make use of the intuitionistic Borel Hierarchy Theorem, established in (Veldman, 1981) and (Veldman, 2001a).
KeywordsNatural Number Infinite Sequence Convergent Sequence Winning Strategy Baire Space
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