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We generalize the definition of an exact sequence of tensor categories due to Brugui`eres and Natale, and introduce a new notion of an exact sequence of (finite) tensor categories with respect to a module category. We give three definitions of this notion and show their equivalence. In particular, the Deligne tensor product of tensor categories gives rise to an exact sequence in our sense. We also show that the dual to an exact sequence in our sense is again an exact sequence. This generalizes the corresponding statement for exact sequences of Hopf algebras. Finally, we show that the middle term of an exact sequence is semisimple if so are the other two terms.
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We give a necessary and sufficient condition in terms of group cohomology for two indecomposable module categories over a group-theoretical fusion category ${mathcal C}$ to be equivalent. This concludes the classification of such module categories.
In this paper, we first introduce $mathcal {W}_F$-Gorenstein modules to establish the following Foxby equivalence: $xymatrix@C=80pt{mathcal {G}(mathcal {F})cap mathcal {A}_C(R) ar@<0.5ex>[r]^{Cotimes_R-} & mathcal {G}(mathcal {W}_F) ar@<0.5ex>[l]^{textrm{Hom}_R(C,-)}} $ where $mathcal {G}(mathcal {F})$, $mathcal {A}_C(R) $ and $mathcal {G}(mathcal {W}_F)$ denote the class of Gorenstein flat modules, the Auslander class and the class of $mathcal {W}_F$-Gorenstein modules respectively. Then, we investigate two-degree $mathcal {W}_F$-Gorenstein modules. An $R$-module $M$ is said to be two-degree $mathcal {W}_F$-Gorenstein if there exists an exact sequence $mathbb{G}_bullet=indent ...longrightarrow G_1longrightarrow G_0longrightarrow G^0longrightarrow G^1longrightarrow...$ in $mathcal {G}(mathcal {W}_F)$ such that $M cong$ $im(G_0rightarrow G^0) $ and that $mathbb{G}_bullet$ is Hom$_R(mathcal {G}(mathcal {W}_F),-)$ and $mathcal {G}(mathcal {W}_F)^+otimes_R-$ exact. We show that two notions of the two-degree $mathcal {W}_F$-Gorenstein and the $mathcal {W}_F$-Gorenstein modules coincide when R is a commutative GF-closed ring.
We present an overview of the notions of exact sequences of Hopf algebras and tensor categories and their connections. We also present some examples illustrating their main features; these include simple fusion categories and a natural question regarding composition series of finite tensor categories.
Let $k$ be an algebraically closed field of characteristic $0$ or $p>2$. Let $mathcal{G}$ be an affine supergroup scheme over $k$. We classify the indecomposable exact module categories over the tensor category ${rm sCoh}_{rm f}(mathcal{G})$ of (coherent sheaves of) finite dimensional $mathcal{O}(mathcal{G})$-supermodules in terms of $(mathcal{H},Psi)$-equivariant coherent sheaves on $mathcal{G}$. We deduce from it the classification of indecomposable {em geometrical} module categories over $sRep(mathcal{G})$. When $mathcal{G}$ is finite, this yields the classification of {em all} indecomposable exact module categories over the finite tensor category $sRep(mathcal{G})$. In particular, we obtain a classification of twists for the supergroup algebra $kmathcal{G}$ of a finite supergroup scheme $mathcal{G}$, and then combine it with cite[Corollary 4.1]{EG3} to classify finite dimensional triangular Hopf algebras with the Chevalley property over $k$.
Let $mathcal{C}$ be a finite braided multitensor category. Let $B$ be Majids automorphism braided group of $mathcal{C}$, then $B$ is a cocommutative Hopf algebra in $mathcal{C}$. We show that the center of $mathcal{C}$ is isomorphic to the category of left $B$-comodules in $mathcal{C}$, and the decomposition of $B$ into a direct sum of indecomposable $mathcal{C}$-subcoalgebras leads to a decomposition of $B$-$operatorname*{Comod}_{mathcal{C}}$ into a direct sum of indecomposable $mathcal{C}$-module subcategories. As an application, we present an explicit characterization of the structure of irreducible Yetter-Drinfeld modules over semisimple quasi-triangular weak Hopf algebras. Our results generalize those results on finite groups and on quasi-triangular Hopf algebras.
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