Number Fields on Fusion Categories

Introduction#

The defining field kk is the base field over which the Hom\text{Hom}-sets of the fusion category C\mathcal{C} are vector spaces, and the tensor product is kk-linear.

Definition

A field extension KEK \subseteq E is called a splitting field for C\mathcal{C} if for every simple object XCX \in \mathcal{C}, the endomorphism algebra EndCK(XK)K\text{End}_{\mathcal{C}_K}(X_K) \cong K, where CK=CEK\mathcal{C}_K = \mathcal{C} \otimes_{E} K is the base change of C\mathcal{C} to KK. In other words, all the simple objects of CK\mathcal{C}_K become "split-simple" over KK.

One often seeks the smallest such field extension KK.