In this paperpaper also the journal front page, eq. 2.14, it introduces the the Grothendieck group $K^0$ of the category of boundary conditions of topological field theory.
My question is that
- what exactly is the Grothendieck group $K^0$ of the category of boundary conditions of topological field theory really mean?
- Does this say that the boundary conditions of topological field theory can be related or classified by a Grothendieck group? Is this an abelian group or nonabelian group? ($K^0(BCondμ(M^{d−1}) )=?$)
- Should the boundary conditions of topological field theory classified by some bimodule of certain modular tensor category? how is this related to a Grothendieck group?