Bimonads and Hopf monads on categories

TL;DR

This paper develops a generalized theory of bimonads and Hopf monads on arbitrary categories, extending classical Hopf algebra concepts to broader categorical contexts using distributive laws and entwining structures.

Contribution

It introduces a framework for bimonads and Hopf monads on any category, utilizing distributive laws and local prebraidings, generalizing Hopf algebra theory beyond vector spaces.

Findings

Defines bimonads as monads and comonads with entwining satisfying certain conditions

Establishes the existence of antipodes as natural transformations with specific properties

Shows the equivalence between antipode existence and category equivalences for categories with limits or colimits

Abstract

The purpose of this paper is to develop a theory of bimonads and Hopf monads on arbitrary categories thus providing the possibility to transfer the essentials of the theory of Hopf algebras in vector spaces to more general settings. There are several extensions of this theory to {\em monoidal} categories which in a certain sense follow the classical trace. Here we do not pose any conditions on our base category but we do refer to the monoidal structure of the category of endofunctors on any category $\A$ and by this we retain some of the combinatorial complexity which makes the theory so interesting. As a basic tool we use distributive laws between monads and comonads (entwinings) on $\A$: we define a {\em bimonad} on $\A$ as an endofunctor $B$ which is a monad and a comonad with an entwining $\lambda:BB\to BB$ satisfying certain conditions. This $\lambda$ is also employed to define the category $\A^B_B$ of (mixed) $B$-bimodules. In the classical situation, an entwining $\lambda$ is derived from the twist map for vector spaces. Here this need not be the case but there may exist special distributive laws $\tau:BB\to BB$ satisfying the Yang-Baxter equation ({\em local prebraidings}) which induce an entwining $\lambda$ and lead to an extension of the theory of {\em braided Hopf algebras}. An antipode is defined as a natural transformation $S:B\to B$ with special properties and for categories $\A$ with limits or colimits and bimonads $B$ preserving them, the existence of an antipode is equivalent to $B$ inducing an equivalence between $\A$ and the category $\A^B_B$ of $B$-bimodules. This is a general form of the {\em Fundamental Theorem} of Hopf algebras.