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Register a new userA 3-categorical perspective on G-crossed braided categories
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A braided monoidal category may be considered a $3$-category with one object and one $1$-morphism. In this paper, we show that, more generally, $3$-categories with one object and $1$-morphisms given by elements of a group $G$ correspond to $G$-crossed braided categories, certain mathematical structures which have emerged as important invariants of low-dimensional quantum field theories. More precisely, we show that the 4-category of $3$-categories $mathcal{C}$ equipped with a 3-functor $mathrm{B}G to mathcal{C}$ which is essentially surjective on objects and $1$-morphisms is equivalent to the $2$-category of $G$-crossed braided categories. This provides a uniform approach to various constructions of $G$-crossed braided categories.
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We develop a method for generating the complete set of basic data under the torsorial actions of $H^2_{[rho]}(G,mathcal{A})$ and $H^3(G,U(1))$ on a $G$-crossed braided tensor category $mathcal{C}_G^times$, where $mathcal{A}$ is the set of invertible simple objects in the braided tensor category $mathcal{C}$. When $mathcal{C}$ is a modular tensor category, the $H^2_{[rho]}(G,mathcal{A})$ and $H^3(G,U(1))$ torsorial action gives a complete generation of possible $G$-crossed extensions, and hence provides a classification. This torsorial classification can be (partially) collapsed by relabeling equivalences that appear when computing the set of $G$-crossed braided extensions of $mathcal{C}$. The torsor method presented here reduces these redundancies by systematizing relabelings by $mathcal{A}$-valued $1$-cochains.
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If $Gamma $ is a group, then braided $Gamma $-crossed modules are classified by braided strict $Gamma $-graded categorial groups. The Schreier theory obtained for $Gamma $-module extensions of the type of an abelian $Gamma $-crossed module is a generalization of the theory of $Gamma $-module extensions.
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