Oracle Computability and Turing Reducibility in the Calculus of Inductive Constructions
This work addresses foundational issues in computability theory for proof assistants, offering incremental advancements in synthetic methods for formal verification.
The paper tackles the problem of defining oracle computability and Turing reducibility in the Calculus of Inductive Constructions (CIC) for machine-checked proofs in Coq, resulting in a synthetic approach using sequential continuity that proves key properties like Turing reducibility forming an upper semilattice and being more expressive than truth-table reducibility.
We develop synthetic notions of oracle computability and Turing reducibility in the Calculus of Inductive Constructions (CIC), the constructive type theory underlying the Coq proof assistant. As usual in synthetic approaches, we employ a definition of oracle computations based on meta-level functions rather than object-level models of computation, relying on the fact that in constructive systems such as CIC all definable functions are computable by construction. Such an approach lends itself well to machine-checked proofs, which we carry out in Coq. There is a tension in finding a good synthetic rendering of the higher-order notion of oracle computability. On the one hand, it has to be informative enough to prove central results, ensuring that all notions are faithfully captured. On the other hand, it has to be restricted enough to benefit from axioms for synthetic computability, which usually concern first-order objects. Drawing inspiration from a definition by Andrej Bauer based on continuous functions in the effective topos, we use a notion of sequential continuity to characterise valid oracle computations. As main technical results, we show that Turing reducibility forms an upper semilattice, transports decidability, and is strictly more expressive than truth-table reducibility, and prove that whenever both a predicate $p$ and its complement are semi-decidable relative to an oracle $q$, then $p$ Turing-reduces to $q$.