Yannick Forster (Gallinette, Nantes), Synthetic Computability in Constructive Type Theory



Mathematical practice in most areas of mathematics is based on the assumption that proofs could be made fully formal in a chosen foundation in principle. This assumption is backed by decades of formalising various areas of mathematics in various proof assistants and various foundations. An area that has been largely neglected for computer-assisted and machine-checked proofs is computability theory. This is due to the fact that making computability theory (and its sibling complexity theory) formal is several orders of magnitude more involved than formalising other areas of mathematics, due to the -- citing Emil Post -- ``forbidding, diverse and alien formalisms in which this [...] work of Gödel, Church, Turing, Kleene, Rosser [...] is embodied.''. For instance, there have been various approaches of formalising Turing machines, all to the ultimate dissatisfaction of the respective authors, and none going further than constructing a universal machine and proving the halting problem undecidable. Professional computability theorist and teachers of computability theory thus rely on the informal Church Turing thesis to carry out their work and only argue the computability of described algorithms informally.

A way out was proposed in the 1980s by Richman and developed during the last decade by Bauer: Synthetic computability theory, where one assumes axioms in a constructive foundation which essentially identify all (constructively definable) functions with computable functions. A drawback of the approach is that assuming such an axiom on top of the axiom of countable choice - which is routinely assumed in this branch of constructive mathematics and computable analysis - is that the law of excluded middle, i.e. classical logic, becomes invalid. Computability theory is however dedicatedly classical: Almost all basic results are presented by appeal to classical axioms and even the full axiom of choice.

We observe that a slight foundational shift rectifies the situation: By basing synthetic computability theory in the Calculus of Inductive Constructions, the type theory underlying amongst others the Coq proof assistant, where countable choice is independent and thus not provable, axioms for synthetic computability are compatible with the law of excluded middle.

I will give an overview over a line of research investigating a synthetic approach to computability theory in constructive type theory, discussing suitable axioms, a Coq library of undecidability proofs, results in the theory of reducibility degrees, a synthetic definition of Kolmogorov complexity, constructive reverse analysis of theorems, and work-in-progress on synthetic oracle computability.

Parts of results are in collaboration with Dominik Kirst, Gert Smolka, Felix Jahn, Fabian Kunze, Nils Lauermann, Niklas Mück, and the contributors of the Coq Library of Undecidability Proofs.