Classical predicative logic-enriched type theories

A logic-enriched type theory (LTT) is a type theory extended with a primitive mechanism for forming and proving propositions. We construct two LTTs, named LTT 0 and LTT 0 ∗ , which we claim correspond closely to the classical predicative systems of second order arithmetic ACA 0 and ACA . We justify this claim by translating each second order system into the corresponding LTT, and proving that these translations are conservative. This is part of an ongoing research project to investigate how LTTs may be used to formalise different approaches to the foundations of mathematics. o LTTs we construct are subsystems of the logic-enriched type theory LTT W , which is intended to formalise the classical predicative foundation presented by Herman Weyl in his monograph Das Kontinuum. The system ACA 0 has also been claimed to correspond to Weyl’s foundation. By casting ACA 0 and ACA as LTTs, we are able to compare them with LTT W . It is a consequence of the work in this paper that LTT W is strictly stronger than ACA 0 . nservativity proof makes use of a novel technique for proving one LTT conservative over another, involving defining an interpretation of the stronger system out of the expressions of the weaker. This technique should be applicable in a wide variety of different cases outside the present work.