Picard Groupoid Research Articles

Green groupoids of 2-Calabi–Yau categories, derived Picard actions, and hyperplane arrangements

We present a construction of (faithful) group actions via derived equivalences in the general categorical setting of algebraic 2-Calabi–Yau triangulated categories. To each algebraic 2-Calabi–Yau category C \mathscr {C} satisfying standard mild assumptions, we associate a groupoid G C \mathscr {G}_{ \mathscr {C} } , named the green groupoid of C \mathscr {C} , defined in an intrinsic homological way. Its objects are given by a set of representatives m r i g C mrig\mathscr {C} of the equivalence classes of basic maximal rigid objects of C \mathscr {C} , arrows are given by mutation, and relations are given by equating monotone (green) paths in the silting order. In this generality we construct a homomorphsim from the green groupoid G C \mathscr {G}_{ \mathscr {C} } to the derived Picard groupoid of the collection of endomorphism rings of representatives of m r i g C mrig\mathscr {C} in a Frobenius model of C \mathscr {C} ; the latter canonically acts by triangle equivalences between the derived categories of the rings. We prove that the constructed representation of the green groupoid G C \mathscr {G}_{ \mathscr {C} } is faithful if the index chamber decompositions of the split Grothendieck groups of basic maximal rigid objects of C \mathscr {C} come from hyperplane arrangements. If Σ 2 ≅ i d \Sigma ^2 \cong id and C \mathscr {C} has finitely many equivalence classes of basic maximal rigid objects, we prove that G C \mathscr {G}_{ \mathscr {C} } is isomorphic to a Deligne groupoid of a hyperplane arrangement and that the representation of this groupoid is faithful.

Categorification of Dijkgraaf–Witten theory

The goal of the paper is to categorify Dijkgraaf-Witten (DW) the- ory, aiming at providing foundation for a direct construction of DW theory as an Extended Topological Quantum Field Theory. The main tool is cohomol- ogy with coefficients in a Picard groupoid, namely the Picard groupoid of hermitian lines.

Abstract representation theory of Dynkin quivers of type A

We study the representation theory of Dynkin quivers of type A in abstract stable homotopy theories, including those associated to fields, rings, schemes, differential-graded algebras, and ring spectra. Reflection functors, (partial) Coxeter functors, and Serre functors are defined in this generality and these equivalences are shown to be induced by universal tilting modules, certain explicitly constructed spectral bimodules. In fact, these universal tilting modules are spectral refinements of classical tilting complexes. As a consequence we obtain split epimorphisms from the spectral Picard groupoid to derived Picard groupoids over arbitrary fields.These results are consequences of a more general calculus of spectral bimodules and admissible morphisms of stable derivators. As further applications of this calculus we obtain examples of universal tilting modules which are new even in the context of representations over a field. This includes Yoneda bimodules on mesh categories which encode all the other universal tilting modules and which lead to a spectral Serre duality result.Finally, using abstract representation theory of linearly oriented An-quivers, we construct canonical higher triangulations in stable derivators and hence, a posteriori, in stable model categories and stable ∞-categories.

The Asaeda–Haagerup fusion categories

Abstract The classification of subfactors of small index revealed several new subfactors. The first subfactor above index 4, the Haagerup subfactor, is increasingly well understood and appears to lie in a (discrete) infinite family of subfactors where the ℤ \mathbb{Z} /3 ℤ \mathbb{Z} symmetry is replaced by other finite abelian groups. The goal of this paper is to give a similarly good description of the Asaeda–Haagerup subfactor which emerged from our study of its Brauer–Picard groupoid. More specifically, we construct a new subfactor 𝒮 {\mathcal{S}} which is a ℤ \mathbb{Z} /4 ℤ \mathbb{Z} × \times ℤ \mathbb{Z} /2 ℤ \mathbb{Z} analogue of the Haagerup subfactor and we show that the even parts of the Asaeda–Haagerup subfactor are higher Morita equivalent to an orbifold quotient of 𝒮 {\mathcal{S}} . This gives a new construction of the Asaeda–Haagerup subfactor which is much more symmetric and easier to work with than the original construction. As a consequence, we can settle many open questions about the Asaeda–Haagerup subfactor: calculating its Drinfeld center, classifying all extensions of the Asaeda–Haagerup fusion categories, finding the full higher Morita equivalence class of the Asaeda–Haagerup fusion categories, and finding intermediate subfactor lattices for subfactors coming from the Asaeda–Haagerup categories. The details of the applications will be given in subsequent papers.

Equivariant gerbes on complex tori

We explore a new direction in representation theory which comes from holomorphic gerbes on complex tori. The analogue of the theta group of a holomorphic line bundle on a (compact) complex torus is developed for gerbes in place of line bundles. The theta group of symmetries of the gerbe has the structure of a Picard groupoid. We calculate it explicitly as a central extension of the group of symmetries of the gerbe by the Picard groupoid of the underlying complex torus. We discuss obstruction to equivariance and give an example of a group of symmetries of a gerbe with respect to which the gerbe cannot be equivariant. We calculate the obstructions to invariant gerbes for some group of translations of a torus to be equivariant. We survey various types of representations of the group of symmetries of a gerbe on the stack of sheaves of modules on the gerbe and the associated abelian category of sheaves on the gerbe (twisted sheaves).

Classification of Invariant Star Products up to Equivariant Morita Equivalence on Symplectic Manifolds

In this paper we investigate equivariant Morita theory for algebras with momentum maps and compute the equivariant Picard groupoid in terms of the Picard groupoid explicitly. We consider three types of Morita theory: ring-theoretic equivalence, *-equivalence, and strong equivalence. Then we apply these general considerations to star product algebras over symplectic manifolds with a Lie algebra symmetry. We obtain the full classification up to equivariant Morita equivalence.

A categorical proof of the Parshin reciprocity laws on algebraic surfaces

We define and study the 2-category of torsors over a Picard groupoid, a central extension of a group by a Picard groupoid, and commutator maps in this central extension. Using it in the context of two-dimensional local fields and two-dimensional adelic theory we obtain the two-dimensional tame symbol and a new proof of Parshin reciprocity laws on an algebraic surface.

Fusion categories and homotopy theory

We apply the yoga of classical homotopy theory to classification problems of G -extensions of fusion and braided fusion categories, where G is a finite group. Namely, we reduce such problems to classification (up to homotopy) of maps from BG to classifying spaces of certain higher groupoids. In particular, to every fusion category \mathcal C we attach the 3-groupoid \underline{\underline{\mathrm{BrPic}}}(\mathcal C) of invertible \mathcal C -bimodule categories, called the Brauer–Picard groupoid of \mathcal C , such that equivalence classes of G -extensions of \mathcal C are in bijection with homotopy classes of maps from BG to the classifying space of \underline{\underline{\mathrm{BrPic}}}(\mathcal C) . This gives rise to an explicit description of both the obstructions to existence of extensions and the data parametrizing them; we work these out both topologically and algebraically. One of the central results of the article is that the 2-truncation of \underline{\underline{\mathrm{BrPic}}}(\mathcal C) is canonically equivalent to the 2-groupoid of braided auto-equivalences of the Drinfeld center \mathcal Z(\mathcal C) of \mathcal C . In particular, this implies that the Brauer–Picard group \mathrm{BrPic}(\mathcal C) (i.e., the group of equivalence classes of invertible \mathcal C -bimodule categories) is naturally isomorphic to the group of braided auto-equivalences of \mathcal Z(\mathcal C) . Thus, if \mathcal C = \mathrm{Vec}_A , where A is a finite abelian group, then \mathrm{BrPic}(\mathcal C) is the orthogonal group \mathrm{O}(A \oplus A^*) . This allows one to obtain a rather explicit classification of extensions in this case; in particular, in the case G = \mathbb Z_2 , we re-derive (without computations) the classical result of Tambara and Yamagami. Moreover, we explicitly describe the category of all (\mathrm{Vec}_{A_1},\mathrm{Vec}_{A_2}) -bimodule categories (not necessarily invertible ones) by showing that it is equivalent to the hyperbolic part of the category of Lagrangian correspondences.

THE COVARIANT PICARD GROUPOID IN DIFFERENTIAL GEOMETRY

In this paper we discuss some general results on the covariant Picard groupoid in the context of differential geometry and interpret the problem of lifting Lie algebra actions to line bundles in the Picard groupoid approach.

Strongly groupoid graded rings and cohomology

We interpret the collection of invertible bimodules as a groupoid and call it the Picard groupoid. We use this groupoid to generalize the classical construction of crossed products to what we call groupoid crossed products, and show that these coincide with the class of strongly groupoid graded rings. We then use groupoid crossed products to obtain a generalization from the group graded situation to the groupoid graded case of the bijection from a second cohomology group, defined by the grading and the functor from the groupoid in question to the Picard groupoid, to the collection of equivalence classes of rings strongly graded by the groupoid.

The H-covariant strong Picard groupoid

The notion of H-covariant strong Morita equivalence is introduced for * -algebras over C = R ( i ) with an ordered ring R which are equipped with a * -action of a Hopf * -algebra H. This defines a corresponding H-covariant strong Picard groupoid which encodes the entire Morita theory. Dropping the positivity conditions one obtains H-covariant * -Morita equivalence with its H-covariant * -Picard groupoid. We discuss various groupoid morphisms between the corresponding notions of the Picard groupoids. Moreover, we realize several Morita invariants in this context as arising from actions of the H-covariant strong Picard groupoid. Crossed products and their Morita theory are investigated using a groupoid morphism from the H-covariant strong Picard groupoid into the strong Picard groupoid of the crossed products.

STATES AND REPRESENTATIONS IN DEFORMATION QUANTIZATION

In this review we discuss various aspects of representation theory in deformation quantization starting with a detailed introduction to the concepts of states as positive functionals and the GNS construction. Rieffel induction of representations as well as strong Morita equivalence, Dirac monopole and strong Picard Groupoid are also discussed.

The Picard Groupoid in Deformation Quantization

In this letter we give an overview on recent developments in representation theory of star product algebras. In particular, we relate the *-representation theory of *-algebras over rings C = R(i) with an ordered ring R and i^2 = -1 to the *-representation theory of *-algebras over \mathbb{C} and point out some properties of the Picard groupoid corresponding to the notion of strong Morita equivalence. Some Morita invariants are interpreted as arising from actions of this groupoid.