Finite quantum hypergroups

TL;DR

This paper develops the theory of finite quantum hypergroups, a class of algebraic structures with a coproduct and integral, clarifying their properties and duality, and providing examples from groups and bicrossproduct constructions.

Contribution

It introduces and clarifies the notion of finite quantum hypergroups, explores their duality, and connects them to algebraic and topological quantum hypergroups, with illustrative examples.

Findings

Finite quantum hypergroups have a dual that is also a finite quantum hypergroup.

The paper clarifies the development of the notion of finite quantum hypergroups.

Examples include structures from finite groups, subgroups, and bicrossproduct theory.

Abstract

A finite quantum hypergroup is a finite-dimensional unital algebra $A$ over the field of complex numbers. There is a coproduct on $A$, a coassociative map from $A$ to $A\otimes A$ assumed to be unital, but it is not required to be an algebra homomorphism. There is a counit that is supposed to be a homomorphism. Finally, the main extra requirement is the existence of a faithful left integral with the right properties. For such a finite quantum hypergroup, the dual can be constructed. It is again a finite quantum hypergroup. The more general concept of an algebraic quantum hypergroup is studied in \cite{De-VD1, De-VD2}. If the underlying algebra of an algebraic quantum hypergroup is finite-dimensional, it is a finite quantum hypergroup in the sense of this paper. Here we treat finite quantum hypergroups independently with an emphasis on the development of the notion. It is meant to clarify the various steps taken in \cite{La-VD3a} and \cite{La-VD3b}. In \cite{La-VD3b} we introduce and study a still more general concept of quantum hypergroups. It not only contains the algebraic quantum hypergroups from \cite{De-VD1}, but also some topological cases. We naturally encounter these quantum hypergroups in our work on bicrossproducts, see \cite{La-VD4, La-VD5, La-VD7}. We include some examples to illustrate the theory. One kind is coming from a finite group and a subgroup. The other examples are taken from the bicrossproduct theory.