The multiplicative unitary as a basis for duality

The classical duality theory associates to an abelian locally compact group a dual companion. Passing to a non-abelian group, a dual object can still be defined, but it is no longer a group. The search for a broader category which should include both the groups and their duals, points towards the concept of quantization. Classically, the regular representation of a group contains the complete information about the structure of this group and its dual. In this article, we follow Baaj and Skandalis and study duality starting from an abstract version of such a representation : the multiplicative unitary. We suggest extra conditions which will replace the regularity and irreducibility of the multiplicative unitary. From the proposed structure of a ”quantum group frame”, we obtain two objects in duality. We equip these objects with certain group-like properties, which make them into candidate quantum groups. We consider the concrete example of the quantum az + b-group, and discuss how it fits into this framework. Finally, we construct the crossed product of a quantum group frame with a locally compact group.