Antoine Allioux (IRIF), Higher Structures in Homotopy Type Theory



The definition of algebraic structures on arbitrary types in homotopy type theory (HoTT) has proven elusive so far. This is due to types behaving like spaces instead of plain sets in general with equalities between elements of a type behaving like homotopies. Equational laws of algebraic structures must therefore be stated coherently. However, in set-based mathematics, the presentation of this coherence data relies on set-level algebraic structures such as operads or presheaves which are thus not subject to additional coherence conditions. Replicating this approach in HoTT leads to a situation of circular dependency as these structures must be defined coherently from the beginning.

We break this situation of circular dependency by extending type theory with a universe of cartesian polynomial monads which, crucially, satisfy their laws definitionally. This extension permits the presentation of types and their higher structures as opetopic types which are infinite collections of cells whose geometry is described by opetopes. Opetopes are geometric shapes originally introduced by Baez and Dolan in order to give a definition of n-categories. The constructors under which our universe of cartesian polynomial monads is closed allow us to define, in particular, the Baez-Dolan slice construction on which is based our definition of opetopic type.

Opetopic types then enable us to define coherent higher algebraic structures, among which infinity-groupoids and (infinity, 1)-categories. Crucially, their higher structure coincides with the one induced by their identity types. We then establish some expected results in order to motivate our definitions.