Alpha-structural recursion and induction

Abstract

The nominal approach to abstract syntax deals with the issues of bound names and α-equivalence by considering constructions and properties that are invariant with respect to permuting names. The use of permutations gives rise to an attractively simple formalization of common, but often technically incorrect uses of structural recursion and induction for abstract syntax modulo α-equivalence. At the heart of this approach is the notion of finitely supported mathematical objects. This article explains the idea in as concrete a way as possible and gives a new derivation within higher-order classical logic of principles of α- structural recursion and induction for α-equivalence classes from the ordinary versions of these principles for abstract syntax trees.