Double Crossproducts of Hopf* Algebras, Discrete Quantum Groups and Amenable Groups

Abstract

In Appendix A, we consider a third construction, the bicrossproduct which is a simultaneous cross product/cross coproduct construction. We find conditions which ensure that the bicrossproduct of two Hopf $\sp{\*}$ algebras is a Hopf $\sp{\*}$ algebra. In Appendix B, we provide physical motivation for studying bicrossproducts by describing a possible theory of quantum gravity where the algebra of quantum observables is a bicrossproduct.