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Sergey Neshveyev

Publications and source records attributed to Sergey Neshveyev.

At least 73 records · Page 4Linked to original sources
arXiv2010

Type III_1 equilibrium states of the Toeplitz algebra of the affine semigroup over the natural numbers

We complete the analysis of KMS-states of the Toeplitz algebra of the affine semigroup over the natural numbers, recently studied by Raeburn and the first author, by showing that for every inverse temperature beta in the critical interval [1,2], the unique KMS_beta-state is of type III_1. We prove this by reducing the type classification from the Toeplitz algebra to that of the symmetric part of the Bost-Connes system, with a shift in inverse temperature. To carry out this reduction we first obtain a parametrization of the Nica spectrum of the Toeplitz algebra in terms of an adelic space. Combining a characterization of traces on crossed products due to the second author with an analysis of the action of the affine semigroup on the Nica spectrum, we can also recover all the KMS-states originally computed by Raeburn and the first author. Our computation sheds light on why there is a free transitive circle action on the extremal KMS_beta-states for beta>2 that does not ostensibly come from an action on the C*-algebra.

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arXiv2009

Von Neumann algebras arising from Bost-Connes type systems

We show that the KMS_beta-states of Bost-Connes type systems for number fields in the region 0<beta\le 1, as well as of the Connes-Marcolli GL_2-system for 1<beta\le 2, have type III_1. This is equivalent to ergodicity of various actions on adelic spaces. For example, the case beta=2 of the GL_2-system corresponds to ergodicity of the action of GL_2(Q) on Mat_2(A) with its Haar measure.

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arXiv2008

Twisted cyclic theory, equivariant KK theory and KMS States

Recently, examples of an index theory for KMS states of circle actions were discovered, \cite{CPR2,CRT}. We show that these examples are not isolated. Rather there is a general framework in which we use KMS states for circle actions on a C*-algebra A to construct Kasparov modules and semifinite spectral triples. By using a residue construction analogous to that used in the semifinite local index formula we associate to these triples a twisted cyclic cocycle on a dense subalgebra of A. This cocycle pairs with the equivariant KK-theory of the mapping cone algebra for the inclusion of the fixed point algebra of the circle action in A. The pairing is expressed in terms of spectral flow between a pair of unbounded self adjoint operators that are Fredholm in the semifinite sense. A novel aspect of our work is the discovery of an eta cocycle that forms a part of our twisted residue cocycle. To illustrate our theorems we observe firstly that they incorporate the results in \cite{CPR2,CRT} as special cases. Next we use the Araki-Woods III_λrepresentations of the Fermion algebra to show that there are examples which are not Cuntz-Krieger systems.

math.OA↗
arXiv2007

Notes on the Kazhdan-Lusztig theorem on equivalence of the Drinfeld category and the category of Uq(g)-modules

We discuss the proof of Kazhdan and Lusztig of the equivalence of the Drinfeld category D(g,h) of g-modules and the category of finite dimensional Uq(g)-modules, q=exp(πih), for h\in C\Q*. Aiming at operator algebraists the result is formulated as the existence for each h\in iR of a normalized unitary 2-cochain F on the dual \hat G of a compact simple Lie group G such that the convolution algebra of G with the coproduct twisted by F is *-isomorphic to the convolution algebra of the q-deformation G_q of G, while the coboundary of F^{-1} coincides with Drinfeld's KZ-associator defined via monodromy of the Knizhnik-Zamolodchikov equations.

math.QA↗
arXiv2007

On Bost-Connes type systems for number fields

We give a complete description of the phase transition of the Bost-Connes type systems for number fields recently introduced by Connes-Marcolli-Ramachandran and Ha-Paugam. We also introduce a notion of K-lattices and discuss an interpretation of these systems in terms of 1-dimensional K-lattices.

math.OA↗
arXiv2007

Hecke algebras of semidirect products and the finite part of the Connes-Marcolli C*-algebra

We study a C*-dynamical system arising from the ring inclusion of the 2\times 2 integer matrices in the rational ones. The orientation preserving affine groups of these rings form a Hecke pair that is closely related to a recent construction of Connes and Marcolli; our dynamical system consists of the associated reduced Hecke C*-algebra endowed with a canonical dynamics defined in terms of the determinant function. We show that the Schlichting completion also consists of affine groups of matrices, over the finite adeles, and we obtain results about the structure and induced representations of the Hecke C*-algebra. In a somewhat unexpected parallel with the one dimensional case studied by Bost and Connes, there is a group of symmetries given by an action of the finite integral ideles, and the corresponding fixed point algebra decomposes as a tensor product over the primes. This decomposition allows us to obtain a complete description of a natural class of equilibrium states which conjecturally includes all KMS_β-states for β\ne 0,1.

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arXiv2007

Phase transition in the Connes-Marcolli GL2-system

We develop a general framework for analyzing KMS-states on C*-algebras arising from actions of Hecke pairs. We then specialize to the system recently introduced by Connes and Marcolli and classify its KMS-states for inverse temperatures β\ne 0,1. In particular, we show that for each β\in(1,2] there exists a unique KMS_β-state.

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arXiv2007

The Dirac operator on compact quantum groups

For the q-deformation G_q, 0<q<1, of any simply connected simple compact Lie group G we construct an equivariant spectral triple which is an isospectral deformation of that defined by the Dirac operator D on G. Our quantum Dirac operator D_q is a unitary twist of D considered as an element of U(g)\otimes Cl(g). The commutator of D_q with a regular function on G_q consists of two parts. One is a twist of a classical commutator and so is automatically bounded. The second is expressed in terms of the commutator of the associator with an extension of D. We show that in the case of the Drinfeld associator the latter commutator is also bounded.

math.OA↗
arXiv2006

The ratio set of the harmonic measure of a random walk on a hyperbolic group

We consider the harmonic measure on the Gromov boundary of a nonamenable hyperbolic group defined by a finite range random walk on the group, and study the corresponding orbit equivalence relation on the boundary. It is known to be always amenable and of type III. We determine its ratio set by showing that it is generated by certain values of the Martin kernel. In particular, we show that the equivalence relation is never of type III_0.

math.DS↗
arXiv2004

Poisson boundary of the dual of SUq(n)

We prove that for any non-trivial product-type action of SUq(n) (0<q<1) on an ITPFI factor N, the relative commutant of the fixed point algebra in N is isomorphic to the algebra of bounded measurable functions on the quantum flag manifold. This is equivalent to the computation of the Poisson boundary of the dual discrete quantum group. The proof relies on a connection between the Poisson integral and the Berezin transform. Our main technical result says that a sequence of Berezin transforms defined by a random walk on the dominant weights of SU(n) converges to the identity on the quantum flag manifold. This is a q-analogue of some known results on quantization of coadjoint orbits of Lie groups.

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arXiv2003

A Local Index Formula for the Quantum Sphere

For the Dirac operator D on the standard quantum sphere we obtain an asymptotic expansion of the SU_q(2)-equivariant entire cyclic cocycle corresponding to εD when evaluated on the element k^2\in U_q(su_2). The constant term of this expansion is a twisted cyclic cocycle which up to a scalar coincides with the volume form and computes the quantum as well as the classical Fredholm indices.

math.QA↗
arXiv2003

Hopf Algebra Equivariant Cyclic Cohomology, K-theory and Index Formulas

For an algebra B with an action of a Hopf algebra H we establish the pairing between even equivariant cyclic cohomology and equivariant K-theory for B. We then extend this formalism to compact quantum group actions and show that equivariant cyclic cohomology is a target space for the equivariant Chern character of equivariant summable Fredholm modules. We prove an analogue of Julg's theorem relating equivariant K-theory to ordinary K-theory of the C*-algebra crossed product, and characterize equivariant vector bundles on quantum homogeneous spaces.

math.KT↗
arXiv2003

KMS states of quasi-free dynamics on Pimsner algebras

A continuous one-parameter group of unitary isometries of a right Hilbert C*-bimodule induces a quasi-free dynamics on the Cuntz-Pimsner C*-algebra of the bimodule and on its Toeplitz extension. The restriction of such a dynamics to the algebra of coefficients of the bimodule is trivial, and the corresponding KMS states of the Toeplitz-Cuntz-Pimsner and Cuntz-Pimsner C*-algebras are characterized in terms of traces on the algebra of coefficients. This generalizes and sheds light onto various earlier results about KMS states of the gauge actions on Cuntz algebras, Cuntz-Krieger algebras, and crossed products by endomorphisms. We also obtain a more general characterization, in terms of KMS weights, for the case in which the inducing isometries are not unitary, and accordingly, the restriction of the quasi-free dynamics to the algebra of coefficients is nontrivial.

math.OA↗
arXiv2001

Ergodic theory and maximal abelian subalgebras of the hyperfinite factor

Let T be a free ergodic measure-preserving action of an abelian group G on (X,mu). The crossed product algebra R_T has two distinguished masas, the image C_T of L^infty(X,mu) and the algebra S_T generated by the image of G. We conjecture that conjugacy of the singular masas S_{T^(1)} and S_{T^(2)} for weakly mixing actions T^(1) and T^(2) of different groups implies that the groups are isomorphic and the actions are conjugate with respect to this isomorphism. Our main result supporting this conjecture is that the conclusion is true under the additional assumption that the isomorphism gamma of R_{T^(1)} onto R_{T^(2)} such that gamma(S_{T^(1)})=S_{T^(2)} has the property that the Cartan subalgebras gamma(C_{T^(1)}) and C_{T^(2)} of R_{T^(2)} are inner conjugate. We discuss a stronger conjecture about the structure of the automorphism group Aut(R_T,S_T), and a weaker one about entropy as a conjugacy invariant. We study also the Pukanszky and some related invariants of S_T, and show that they have a simple interpretation in terms of the spectral theory of the action T. It follows that essentially all values of the Pukanszky invariant are realized by the masas S_T, and there exist non-conjugate singular masas with the same Pukanszky invariant.

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arXiv2000

The variational principle for a class of asymptotically abelian C*-algebras

Let (A,α) be a C*-dynamical system. We introduce the notion of pressure P_α(H) of the automorphism αat a self-adjoint operator H\in A. Then we consider the class of AF-systems satisfying the following condition: there exists a dense α-invariant *-subalgebra \A of A such that for all pairs a,b\in\A the C*-algebra they generate is finite dimensional, and there is p=p(a,b)\in\N such that [α^j(a),b]=0 for |j|\ge p. For systems in this class we prove the variational principle, i.e. show that P_α(H) is the supremum of the quantities h_ϕ(α)-ϕ(H), where h_ϕ(α) is the Connes-Narnhofer-Thirring dynamical entropy of αwith respect to the α-invariant state ϕ. If H\in\A, and P_α(H) is finite, we show that any state on which the supremum is attained is a KMS-state with respect to a one-parameter automorphism group naturally associated with H. In particular, Voiculescu's topological entropy is equal to the supremum of h_ϕ(α), and any state of finite maximal entropy is a trace.

math.OA↗
arXiv2000

Entropy of automorphisms of II_1-factors arising from the dynamical systems theory

Let a countable amenable group G acts freely and ergodically on a Lebesgue space (X,mu), preserving the measure mu. If T is an automorphism of the equivalence relation defined by G then T can be extended to an automorphism alpha_T of the II_1-factor M=L^\infty(X,μ)\rtimes G. We prove that if T commutes with the action of G then H(alpha_T)=h(T), where H(alpha_T) is the Connes- Stormer entropy of alpha_T, and h(T) is the Kolmogorov-Sinai entropy of T. We prove also that for given s and t, 0\le s\le t\le\infty, there exists a T such that h(T)=s and H(alpha_T)=t.

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