On Split Forms of Fusion Categories

Abstract

We formulate split Galois descent for fusion categories through framed equivalences, reducing the existence of a split form to a group-theoretic splitting problem. The associated degree-two obstruction combines categorical coherence with stable descent of the simple objects, while fusion spaces impose index and parity restrictions. For quantum groups at roots of unity, we use the quasi-$R$-matrix to make the bar involution tensor-compatible and prove that the underlying fusion category $\mathcal{C}(\mathfrak{g},k)$, associated with the simply connected root datum, has a split form over the maximal totally real subfield of the cyclotomic field generated by the quantum parameter. The same holds for all its cyclotomic Galois conjugates; in particular, every $\mathcal{C}(\mathfrak{g},k)$ has a split real form. By contrast, no split real form exists for the standard braiding except on $\mathcal{C}(E_8,1)$ and $\mathcal{C}(D_{4m},1)$, $m\ge 1$. We further show that associative zesting can destroy real descent of $\mathcal{C}(\mathfrak{g},k)$, while every braided zesting of $\mathcal{C}(\mathfrak{g},k)$ still admits a split real form as an underlying fusion category. Applications to pointed and finite-group representation categories give criteria for real descent in terms of cohomology and Frobenius--Schur indicators.

49 pages