arXiv subjects

Daniël Otten

Publications and source records attributed to Daniël Otten.

4 recordsLinked to original sources

arXiv2026

Constructing (Co)inductive Types via Large Sizes

To ensure decidability and consistency of its type theory, a proof assistant should only accept terminating recursive functions and productive corecursive functions. Most proof assistants enforce this through syntactic conditions, which can be restrictive and non-modular. Sized types are a type-based alternative where (co)inductive types are annotated with additional size information. Well-founded induction on sizes can then be used to prove termination and productivity. An implementation of sized types exists in Agda, but it is currently inconsistent due to the addition of a largest size. We investigate an alternative approach, where intensional type theory is extended with a large type of sizes and parametric quantifiers over sizes. We show that inductive and coinductive types can be constructed in this theory, which improves on earlier work where this was only possible for the finitely-branching inductive types. The consistency of the theory is justified by an impredicative realisability model, which interprets the type of sizes as an uncountable ordinal.

cs.LO↗

arXiv2026

Unravelling Abstract Cyclic Proofs into Proofs by Induction

Cyclic proof theory breaks tradition by allowing certain infinite proofs: those that can be represented by a finite graph, while satisfying a soundness condition. We reconcile cyclic proofs with traditional finite proofs: we extend abstract cyclic proof systems with a well-founded induction principle, and transform any cyclic proof into a finite proof in the extended system. Moreover, this transformation preserves the structure of the cyclic proof. Our results leverage an annotated representation of cyclic proofs, which allows us to extract induction hypotheses and to determine their introduction order. The representation is essentially a reset proof with one key modification: names must be covered in a uniform way before a reset. This innovation allows us to handle cyclic proofs where the underlying inductive sort is non-linear. Our framework is general enough to cover recursive functions satisfying the size-change termination principle, which are viewed as cyclic proofs under the Curry-Howard correspondence.

cs.LO↗

arXiv2025

The biequivalence of path categories and axiomatic Martin-Löf type theories

The semantics of extensional type theory has an elegant categorical description: models of extensional =-types, 1-types, and Sigma-types are biequivalent to finitely complete categories, while adding Pi-types yields locally Cartesian closed categories. We establish parallel results for axiomatic type theory, which includes systems like cubical type theory, where the computation rule of the =-types only holds as a propositional axiom instead of a definitional reduction. In particular, we prove that models of axiomatic =-types, and standard 1- and Sigma-types are biequivalent to certain path categories, while adding axiomatic Pi-types yields dependent homotopy exponents. This biequivalence simplifies axiomatic =-types, which are more intricate than extensional ones since they permit higher dimensional structure. Specifically, path categories use a primitive notion of equivalence instead of a direct reproduction of the syntactic elimination rules and computation axioms. We apply our correspondence to prove a coherence theorem: we show that these weak homotopical models can be turned into equivalent strict models of axiomatic type theory. In addition, we introduce a more modular notion, that of a display map path category, which only models axiomatic =-types by default, while leaving room to add other axiomatic type formers such as 1-, Sigma-, and Pi-types.

math.LO↗

arXiv2023

Conservativity of Type Theory over Higher-order Arithmetic

We investigate how much type theory is able to prove about the natural numbers. A classical result in this area shows that dependent type theory without any universes is conservative over Heyting Arithmetic (HA). We build on this result by showing that type theories with one level of universes are conservative over Higher-order Heyting Arithmetic (HAH). Although this clearly depends on the specific type theory, we show that the interpretation of logic also plays a major role. For proof-irrelevant interpretations, we will see that strong versions of type theory prove exactly the same higher-order arithmetical formulas as HAH. Conversely, proof-relevant interpretations prove strictly more second-order arithmetical formulas than HAH, however they still prove exactly the same first-order arithmetical formulas. Along the way, we investigate the different interpretations of logic in type theory, and to what extent dependent type theories can be seen as extensions of higher-order logic. We apply our results by proving De Jongh's theorem for type theory.

math.LO↗