Intuitionism and effective descriptive set theory

Abstract

Our very eloquent charge from Jan van Mill was to “draw a line to Brouwer from descriptive set theory, but this proved elusive: in fact there are few references to Brouwer, in Lusin (1928) [36] and Lusin (1930), none of them substantial; and even though Brouwer refers to Borel, Lebesgue and Hadamard in his early papers, it does not appear that he was influenced by their work in any substantive way.11Cf. Michel(2008) [40], who traces the origin and dismisses the significance of terms like “semi-” or “pre-” intuitionists.We have not found any references by him to more developed work in descriptive set theory, after the critical Lebesgue (1905). So instead of looking for historical connections or direct influences (in either direction), we decided to identify and analyze some basic themes, problems and results which are common to these two fields; and, as it turns out, the most significant connections are between intuitionistic analysis and effective descriptive set theory, hence the title.⇒ We will outline our approach and (limited) aims in Section 1, marking with an arrow (like this one) those paragraphs which point to specific parts of the article. Suffice it to say here that our main aim is to identify a few, basic results of descriptive set theory which can be formulated and justified using principles that are both intuitionistically and classically acceptable; that we will concentrate on the mathematics of the matter rather than on history or philosophy; and that we will use standard, classical terminology and notation.This is an elementary, mostly expository paper, broadly aimed at students of logic and set theory who also know the basic facts about recursive functions but need not know a lot about either intuitionism or descriptive set theory. The only (possibly) new result is Theorem 6.1, which justifies simple definitions and proofs by induction in Kleene’s Basic System of intuitionistic analysis, and is then used inTheorems 7.1 and 7.2 to give in the same system a rigorous definition of the Borel sets and prove that they are analytic; the formulation and proof of this last result is one example where methods from effective descriptive set theory are used in an essential way.