arXiv subjects

Sergey Neshveyev

Publications and source records attributed to Sergey Neshveyev.

At least 55 records · Page 3Linked to original sources

arXiv2015

Quantum flag manifolds as quotients of degenerate quantized universal enveloping algebras

Let $\mathfrak{g}$ be a semi-simple Lie algebra with fixed root system, and $U_q(\mathfrak{g})$ the quantization of its universal enveloping algebra. Let $\mathcal{S}$ be a subset of the simple roots of $\mathfrak{g}$. We show that the defining relations for $U_q(\mathfrak{g})$ can be slightly modified in such a way that the resulting algebra $U_q(\mathfrak{g};\mathcal{S})$ allows a homomorphism onto (an extension of) the algebra $\mathrm{Pol}(\mathbb{G}_q/\mathbb{K}_{\mathcal{S},q})$ of functions on the quantum flag manifold $\mathbb{G}_q/\mathbb{K}_{\mathcal{S},q}$ corresponding to $\mathcal{S}$. Moreover, this homomorphism is equivariant with respect to a natural adjoint action of $U_q(\mathfrak{g})$ on $U_q(\mathfrak{g};\mathcal{S})$ and the standard action of $U_q(\mathfrak{g})$ on $Pol(\mathbb{G}_q/\mathbb{K}_{\mathcal{S},q})$.

math.QA↗

arXiv2015

von Neuman algebras of strongly connected higher-rank graphs

We investigate the factor types of the extremal KMS states for the preferred dynamics on the Toeplitz algebra and the Cuntz--Krieger algebra of a strongly connected finite $k$-graph. For inverse temperatures above 1, all of the extremal KMS states are of type I$_\infty$. At inverse temperature 1, there is a dichotomy: if the $k$-graph is a simple $k$-dimensional cycle, we obtain a finite type I factor; otherwise we obtain a type III factor, whose Connes invariant we compute in terms of the spectral radii of the coordinate matrices and the degrees of cycles in the graph.

math.OA↗

arXiv2014

Categorical duality for Yetter-Drinfeld algebras

We study tensor structures on (Rep G)-module categories defined by actions of a compact quantum group G on unital C*-algebras. We show that having a tensor product which defines the module structure is equivalent to enriching the action of G to the structure of a braided-commutative Yetter-Drinfeld algebra. This shows that the category of braided-commutative Yetter-Drinfeld G-C*-algebras is equivalent to the category of generating unitary tensor functors from Rep G into C*-tensor categories. To illustrate this equivalence, we discuss coideals of quotient type in C(G), Hopf-Galois extensions and noncommutative Poisson boundaries.

math.OA↗

arXiv2014

KMS states on the C*-algebras of non-principal groupoids

We describe KMS-states on the C*-algebras of etale groupoids in terms of measurable fields of traces on the C*-algebras of the isotropy groups. We use this description to analyze tracial states on the transformation groupoid C*-algebras and to give a short proof of recent results of Cuntz, Deninger and Laca on the Toeplitz algebras of the ax+b semigroups of the rings of integers in number fields.

math.OA↗

arXiv2014

Bost-Connes systems associated with function fields

With a global function field K with constant field F_q, a finite set S of primes in K and an abelian extension L of K, finite or infinite, we associate a C*-dynamical system. The systems, or at least their underlying groupoids, defined earlier by Jacob using the ideal action on Drinfeld modules and by Consani-Marcolli using commensurability of K-lattices are isomorphic to particular cases of our construction. We prove a phase transition theorem for our systems and show that the unique KMS_β-state for every 0<β\le1 gives rise to an ITPFI-factor of type III_{q^{-βn}}, where n is the degree of the algebraic closure of F_q in L. Therefore for n=+\infty we get a factor of type III_0. Its flow of weights is a scaled suspension flow of the translation by the Frobenius element on Gal(\bar F_q/F_q).

math.OA↗

arXiv2013

Deformation of C*-algebras by cocycles on locally compact quantum groups

Given a C*-algebra A with a left action of a locally compact quantum group G on it and a unitary 2-cocycle Omega on \hat G, we define a deformation A_Omega of A. The construction behaves well under certain additional technical assumptions on Omega, the most important of which is regularity, meaning that C_0(G)_Omega\rtimes G is isomorphic to the algebra of compact operators on some Hilbert space. In particular, then A_Ωis stably isomorphic to the iterated twisted crossed product \hat G^{op}\ltimes_ΩG\ltimes A. Also, in good situations, the C*-algebra A_Ωcarries a left action of the deformed quantum group G_Ωand we have an isomorphism G_Ω\ltimes A_Ω\cong G\ltimes A. When G is a genuine locally compact group, we show that the action of G on C_0(G)_Omega=C*_r(\hat G;Omega) is always integrable. Stronger assumptions of properness and saturation of the action imply regularity. As an example, we make a preliminary analysis of the cocycles on the duals of some solvable Lie groups recently constructed by Bieliavsky et al., and discuss the relation of our construction to that of Bieliavsky and Gayral.

math.OA↗

arXiv2013

Smooth crossed products of Rieffel's deformations

Assume A is a Frechet algebra equipped with a smooth isometric action of a vector group V, and consider Rieffel's deformation A_J of A. We construct an explicit isomorphism between the smooth crossed products V\ltimes\A_J and V\ltimes\A. When combined with the Elliott-Natsume-Nest isomorphism, this immediately implies that the periodic cyclic cohomology is invariant under deformation. Specializing to the case of smooth subalgebras of C*-algebras, we also get a simple proof of equivalence of Rieffel's and Kasprzak's approaches to deformation.

math.OA↗

arXiv2013

Twisting the q-deformations of compact semisimple Lie groups

Given a compact semisimple Lie group $G$ of rank $r$, and a parameter $q>0$, we can define new associativity morphisms in Rep(Gq) using a 3-cocycle $Φ$ on the dual of the center of G, thus getting a new tensor category Rep(Gq)$^Φ$. For a class of cocycles $Φ$ we construct compact quantum groups $G^τ_q$ with representation categories Rep(Gq)$^Φ$. The construction depends on the choice of an r-tuple $τ$ of elements in the center of G. In the simplest case of G=SU(2) and $τ=-1$, our construction produces Woronowicz's quantum group SU_{-q}(2) out of SUq(2). More generally, for G=SU(n), we get quantum group realizations of the Kazhdan-Wenzl categories.

math.OA↗

arXiv2013

Ergodicity of the action of K* on A_K

Connes gave a spectral interpretation of the critical zeros of zeta- and L-functions for a global field K using a space of square integrable functions on the space A_K/K* of adele classes. It is known that for K=Q the space A_K/K* cannot be understood classically, or in other words, the action of Q* on A_Q is ergodic. We prove that the same is true for any global field K, in both the number field and function field cases.

math.NT↗

arXiv2013

Deformation of operator algebras by Borel cocycles

Assume that we are given a coaction δof a locally compact group G on a C*-algebra A and a T-valued Borel 2-cocycle ωon G. Motivated by the approach of Kasprzak to Rieffel's deformation we define a deformation A_ωof A. Among other properties of A_ωwe show that A_ω\otimes K(L^2(G)) is canonically isomorphic to A\rtimes_δ\hat G\rtimes_{\hatδ,ω}G. This, together with a slight extension of a result of Echterhoff et al., implies that for groups satisfying the Baum-Connes conjecture the K-theory of A_ωremains invariant under homotopies of omega.

math.OA↗

arXiv2013

Autoequivalences of the tensor category of Uq(g)-modules

We prove that for q\in\C* not a nontrivial root of unity the cohomology group defined by invariant 2-cocycles in a completion of Uq(g) is isomorphic to H^2(P/Q;\T), where P and Q are the weight and root lattices of g. This implies that the group of autoequivalences of the tensor category of Uq(g)-modules is the semidirect product of H^2(P/Q;\T) and the automorphism group of the based root datum of g. For q=1 we also obtain similar results for all compact connected separable groups.

math.QA↗

arXiv2013

Bost-Connes systems, Hecke algebras, and induction

We consider a Hecke algebra naturally associated with the affine group with totally positive multiplicative part over an algebraic number field K and we show that the C*-algebra of the Bost-Connes system for K can be obtained from our Hecke algebra by induction, from the group of totally positive principal ideals to the whole group of ideals. Our Hecke algebra is therefore a full corner, corresponding to the narrow Hilbert class field, in the Bost-Connes C*-algebra of K; in particular, the two algebras coincide if and only if K has narrow class number one. Passing the known results for the Bost-Connes system for K to this corner, we obtain a phase transition theorem for our Hecke algebra. In another application of induction we consider an extension L/K of number fields and we show that the Bost-Connes system for L embeds into the system obtained from the Bost-Connes system for K by induction from the group of ideals in K to the group of ideals in L. This gives a C*-algebraic correspondence from the Bost-Connes system for K to that for L. Therefore the construction of Bost-Connes systems can be extended to a functor from number fields to C*-dynamical systems with equivariant correspondences as morphisms. We use this correspondence to induce KMS-states and we show that for beta>1 certain extremal KMS_beta-states for L can be obtained, via induction and rescaling, from KMS_{[L:K]beta}-states for K. On the other hand, for 0<beta\le1 every KMS_{[L:K]β}-state for K induces to an infinite weight.

math.OA↗

arXiv2013

Duality theory for nonergodic actions

Generalizing work by Pinzari and Roberts, we characterize actions of a compact quantum group G on C*-algebras in terms of what we call weak unitary tensor functors from Rep G into categories of C*-correspondences. We discuss the relation of our construction of a C*-algebra from a functor to some well-known crossed product type constructions, such as cross-sectional algebras of Fell bundles and crossed products by Hilbert bimodules. We also relate our setting to recent work of De Commer and Yamashita by showing that any object in a module C*-category over Rep G produces a weak unitary tensor functor, and, as a consequence, actions can also be described in terms of (Rep G)-module C*-categories. As an application we discuss deformations of C*-algebras by cocycles on discrete quantum groups.

math.OA↗

arXiv2011

On second cohomology of duals of compact groups

We show that for any compact connected group G the second cohomology group defined by unitary invariant 2-cocycles on \hat G is canonically isomorphic to H^2(\hat{Z(G)};T). This implies that the group of autoequivalences of the C*-tensor category Rep G is isomorphic to H^2(\hat{Z(G)};T)\rtimes\Out(G). We also show that a compact connected group G is completely determined by Rep G. More generally, extending a result of Etingof-Gelaki and Izumi-Kosaki we describe all pairs of compact separable monoidally equivalent groups. The proofs rely on the theory of ergodic actions of compact groups developed by Landstad and Wassermann and on its algebraic counterpart developed by Etingof and Gelaki for the classification of triangular semisimple Hopf algebras. In two appendices we give a self-contained account of amenability of tensor categories, fusion rings and discrete quantum groups, and prove an analogue of Radford's theorem on minimal Hopf subalgebras of quasitriangular Hopf algebras for compact quantum groups.

math.OA↗

arXiv2011

Quantized algebras of functions on homogeneous spaces with Poisson stabilizers

Let G be a simply connected semisimple compact Lie group with standard Poisson structure, K a closed Poisson-Lie subgroup, 0<q<1. We study a quantization C(G_q/K_q) of the algebra of continuous functions on G/K. Using results of Soibelman and Dijkhuizen-Stokman we classify the irreducible representations of C(G_q/K_q) and obtain a composition series for C(G_q/K_q). We describe closures of the symplectic leaves of G/K refining the well-known description in the case of flag manifolds in terms of the Bruhat order. We then show that the same rules describe the topology on the spectrum of C(G_q/K_q). Next we show that the family of C*-algebras C(G_q/K_q), 0<q\le1, has a canonical structure of a continuous field of C*-algebras and provides a strict deformation quantization of the Poisson algebra \C[G/K]. Finally, extending a result of Nagy, we show that C(G_q/K_q) is canonically KK-equivalent to C(G/K).

math.OA↗

arXiv2011

K-homology class of the Dirac operator on a compact quantum group

By a result of Nagy, the C*-algebra of continuous functions on the q-deformation G_q of a simply connected semisimple compact Lie group G is KK-equivalent to C(G). We show that under this equivalence the K-homology class of the Dirac operator on G_q, which we constructed in an earlier paper, corresponds to that of the classical Dirac operator. Along the way we prove that for an appropriate choice of isomorphisms between completions of U_q(g) and U(g) a family of Drinfeld twists relating the deformed and classical coproducts can be chosen to be continuous in q.

math.OA↗

arXiv2011

Symmetric invariant cocycles on the duals of q-deformations

We prove that for q not a nontrivial root of unity any symmetric invariant 2-cocycle for a completion of Uq(g) is the coboundary of a central element. Equivalently, a Drinfeld twist relating the coproducts on completions of Uq(g) and U(g) is unique up to coboundary of a central element. As an application we show that the spectral triple we defined in an earlier paper for the q-deformation of a simply connected semisimple compact Lie group G does not depend on any choices up to unitary equivalence.

math.QA↗

arXiv2010

Traces on crossed products

We give a description of traces on C(X)\rtimes G in terms of measurable fields of traces on the C*-algebras of the stabilizers of the action of G on X.

math.OA↗