Open sets in computability theory and reverse mathematics

Abstract

Abstract To enable the study of open sets in computational approaches to mathematics, lots of extra data and structure on these sets is assumed. For both foundational and mathematical reasons, it is then a natural question, and the subject of this paper, what the influence of this extra data and structure is on the logical and computational properties of basic theorems pertaining to open sets. To answer this question, we study various basic theorems of analysis, like the Baire category, Heine, Heine–Borel, Urysohn and Tietze theorems, all for open sets given by their (third-order) characteristic functions. Regarding computability theory, the objects claimed to exist by the aforementioned theorems undergo a shift from ‘computable’ to ‘not computable in any type 2 functional’, following Kleene’s S1–S9. Regarding reverse mathematics, the latter’s main question, namely which set existence axioms are necessary for proving a given theorem, does not have a unique or unambiguous answer for the aforementioned theorems, working in Kohlenbach’s higher-order framework. A finer study of representations of open sets leads to the new ‘$\varDelta$-functional’ that has unique (computational) properties.