Zigzag normalisation for associative n-categories

Abstract

The theory of associative n-categories has recently been proposed as a strictly associative and unital approach to higher category theory. As a foundation for a proof assistant, this is potentially attractive, since it has the potential to allow simple formal proofs of complex high-dimensional algebraic phenomena. However, the theory relies on an implicit term normalisation procedure to recognize correct composites, with no recursive method available for computing it. Here we describe a new approach to term normalisation in associative n-categories, based on the categorical zigzag construction. This radically simplifies the theory, and yields a recursive algorithm for normalisation, which we prove is correct. Our use of categorical lifting properties allows us to give efficient proofs of our results. Our normalisation algorithm forms a core component of the proof assistant homotopy.io, and we illustrate our scheme with worked examples.