Cyclic proofs, system t, and the power of contraction

We study a cyclic proof system C over regular expression types, inspired by linear logic and non-wellfounded proof theory. Proofs in C can be seen as strongly typed goto programs. We show that they denote computable total functions and we analyse the relative strength of C and Gödel’s system T. In the general case, we prove that the two systems capture the same functions on natural numbers. In the affine case, i.e., when contraction is removed, we prove that they capture precisely the primitive recursive functions—providing an alternative and more general proof of a result by Dal Lago, about an affine version of system T. Without contraction, we manage to give a direct and uniform encoding of C into T, by analysing cycles and translating them into explicit recursions. Whether such a direct and uniform translation from C to T can be given in the presence of contraction remains open. We obtain the two upper bounds on the expressivity of C using a different technique: we formalise weak normalisation of a small step reduction semantics in subsystems of second-order arithmetic: ACA0 and RCA0.