Alfons Van Daele

UNIVERSAL QUANTUM GROUPS

For each invertible m×m matrix Q a compact matrix quantum group Au(Q) is constructed. These quantum groups are shown to be universal in the sense that any compact matrix quantum group is a quantum subgroup of some of them. Their orthogonal version Ao(Q) is also constructed. Finally, we discuss related constructions in the literature.

ACTIONS OF MULTIPLIER HOPF ALGEBRAS

For an action a of a group G on an algebra R (over C), the crossed product Rxα G is the vector space of R-valued functions with finite support in G, together with the twisted convolution product given by where p∈G. This construction has been extended to the theory of Hopf algebras. Given an action of a Hopf algebra A on an algebra R, it is possible to make the tensor productR⊗A into an algebra by using a twisted product, involving the action. In this case, the algebra is called the smash product and denoted by R#A. In the group case, the action a of G on R yields an action of the group algebra CG as a Hopf algebra on R and the crossed Rxα G coincides with the smash product R#CG. In this paper we extend the theory of actions of Hopf algebras to actions of multiplier Hopf algebras. We also construct the smash product and we obtain results very similar as in the original situation for Hopf algebras. The main result in the paper is a duality theorem for such actions. We consider dual pairs of multiplier Hopf ...

Pairing and Quantum Double of Multiplier Hopf Algebras

We define and investigate pairings of multiplier Hopf (*-)algebras which are nonunital generalizations of Hopf algebras. Dual pairs of multiplier Hopf algebras arise naturally from any multiplier Hopf algebra A with integral and its dual Â. Pairings of multiplier Hopf algebras play a basic rôle, e.g., in the study of actions and coactions, and, in particular, in the relation between them. This aspect of the theory is treated elsewhere. In this paper we consider the quantum double construction out of a dual pair of multiplier Hopf algebras. We show that two dually paired regular multiplier Hopf (*-)algebras A and B yield a quantum double which is again a regular multiplier Hopf (*-)algebra. If A and B have integrals, then the quantum double also has an integral. If A and B are Hopf algebras, then the quantum double multiplier Hopf algebra is the usual quantum double. The quantum double construction for dually paired multiplier Hopf (*-)algebras yields new nontrivial examples of multiplier Hopf (*-)algebras.

The Larson–Sweedler theorem for weak multiplier Hopf algebras

ABSTRACT The Larson–Sweedler theorem says that a finite-dimensional bialgebra with a faithful integral is a Hopf algebra [15]. The result has been generalized to finite-dimensional weak Hopf algebras by Vecsernyés [44]. In this paper, we show that the result is still true for weak multiplier Hopf algebras. The notion of a weak multiplier bialgebra was introduced by Böhm et al. in [4]. In this note it is shown that a weak multiplier bialgebra with a regular and full coproduct is a regular weak multiplier Hopf algebra if there is a faithful set of integrals. Weak multiplier Hopf algebras are introduced and studied in [40]. Integrals on (regular) weak multiplier Hopf algebras are treated in [43]. This result is important for the development of the theory of locally compact quantum groupoids in the operator algebra setting, see [13] and [14]. Our treatment of this material is motivated by the prospect of such a theory.

Semi-invariants of actions of multiplier hopf algebras

Let A be a multiplier Hopf algebra which acts on an algebra R. In this paper we study semi-invariants of this action. This idea has proved interesting in the case thatA is a Hopf algebra.

Multiplier Hopf algebroids: Basic theory and examples

ABSTRACT Multiplier Hopf algebroids are algebraic versions of quantum groupoids that generalize Hopf algebroids to the non-unital case and weak (multiplier) Hopf algebras to non-separable base algebras. The main structure maps of a multiplier Hopf algebroid are a left and a right comultiplication. We show that bijectivity of two associated canonical maps is equivalent to the existence of an antipode, discusses invertibility of the antipode, and presents some examples and special cases.