Tensor product of quantum groups was defined by Wang in [Wan95] generalizing the group direct product.
Let and be groups. Then we can construct their direct product
with group operation . We can then identify the group with a subgroup and similarly . Then we can say that the elements of commute with the elements of in in the sense that . In addition, the groups and can also be obtained as quotient groups of .
If and are compact matrix groups with fundamental representations and , then is also a compact group that can be represented by the direct sum . For the associated C*-algebra of continuous functions, we have as this holds for any compact spaces. We can also have a look on the comultiplication on . Take and and take , then
so .
Nevertheless, the direct product is defined for any pair of groups, not only the compact ones. In particular, if and are discrete groups, then defines a discrete group. For the associated group algebras, we obviously have and hence .
Let and be compact quantum groups. We define [Wan95] their tensor product to be the quantum group with underlying C*-algebra and comultiplication defined as
Formally, we should rather write , where and are the canonical inclusions.
The above mentioned construction indeed defines a compact quantum group .
Taking two compact groups and , their quantum group tensor product coincides with the group direct product.
Considering an element , we usually omit the sign . One way to view this is to consider as a quotient of with respect to the relations for and . The other viewpoint is to consider and as subalgebras of . This also shows that and can be considered as quotient quantum groups of . In addition, we also have that and are quotients of , so and are quantum subgroups of .
If and are compact matrix quantum groups, we can define the structure of a compact matrix quantum group on .
Let and be compact matrix quantum groups. Then
is also a compact matrix quantum group. It is a matrix realization of the tensor product of and as defined above.
Let and be compact quantum groups. Let and be complete sets of irreducible representation of and . Denote by and the embeddings of and into , respectively, and denote and . Then a complete set of irreducible representations of is formed by with , [Wan95].
Let and be compact quantum groups and , the corresponding Haar states. Then the Haar state of is of the form [Wan95].