Lax Monoidal Fibrations

TL;DR

This paper introduces lax monoidal fibrations and demonstrates their utility in modeling algebraic structures relevant to opetopic sets, establishing equivalences between various fibrations and extending the theory to symmetric signatures and analytic functors.

Contribution

It develops the concept of lax monoidal fibrations and applies it to unify and extend the theory of opetopic sets, multicategories, and polynomial and analytic monads.

Findings

Fibration of T-categories is formed by monoids in Burroni lax monoidal fibrations.

Fibration of multicategories is equivalent to the fibration of polynomial monads.

Fibrations of symmetric multicategories are equivalent to fibrations of analytic monads.

Abstract

We introduce the notion of a lax monoidal fibration and we show how it can be conveniently used to deal with various algebraic structures that play an important role in some definitions of the opetopic sets (Baez-Dolan, Hermida-Makkai-Power). We present the 'standard' such structures, the exponential fibrations of basic fibrations and three areas of applications. First area is related to the T-categories of A. Burroni. The monoids in the Burroni lax monoidal fibrations form the fibration of T-categories. The construction of the relative Burroni fibrations and free T-categories in this context, allow us to extend the definition of the set of opetopes given by T. Leinster to the category of opetopic sets (internally to any Grothendieck topos, if needed). We also show that fibration of (1-level) multicategories, considered by Hermida-Makkai-Power, is equivalent to the fibration of (finitary, cartesian) polynomial monads. This equivalence is induced by the equivalence of lax monoidal fibrations of amalgamated signatures, polynomial diagrams, and polynomial (finitary, endo) functors. Finally, we develop a similar theory for symmetric signatures, analytic diagrams (a notion introduced here), and (finitary, multivariable) analytic (endo)functors. Among other things we show that the fibrations of symmetric multicategories is equivalent to the fibration of analytic monads. We also give a characterization of such a fibration of analytic monads. An object of this fibration is a weakly cartesian monad on a slice of Set whose functor parts is a finitary functors weakly preserving wide pullbacks. A morphism of this fibration is a weakly cartesian morphism of monads whose functor part is a pullback functor.