arXiv subjects

David Kyed

Publications and source records attributed to David Kyed.

30 records · Page 2Linked to original sources

arXiv2013

Measure continuous derivations on von Neumann algebras and applications to L^2-cohomology

We prove that norm continuous derivations from a von Neumann algebra into the algebra of operators affiliated with its tensor square are automatically continuous for both the strong operator topology and the measure topology. Furthermore, we prove that the first continuous L^2-Betti number scales quadratically when passing to corner algebras and derive an upper bound given by Shen's generator invariant. This, in turn, yields vanishing of the first continuous L^2-Betti number for II_1 factors with property (T), for finitely generated factors with non-trivial fundamental group and for factors with property Gamma.

math.OA↗

arXiv2013

A groupoid approach to Lück's amenability conjecture

We prove that amenability of a discrete group is equivalent to dimension flatness of certain ring inclusions naturally associated with measure preserving actions of the group. This provides a group-measure space theoretic solution to a conjecture of Lück stating that amenability of a group is characterized by dimension flatness of the inclusion of its complex group algebra into the associated von Neumann algebra.

math.GR↗

arXiv2013

L^2-Betti numbers of locally compact groups and their cross section equivalence relations

We prove that the L^2-Betti numbers of a unimodular locally compact group G coincide, up to a natural scaling constant, with the L^2-Betti numbers of the countable equivalence relation induced on a cross section of any essentially free ergodic probability measure preserving action of G. As a consequence, we obtain that the reduced and un-reduced L^2-Betti numbers of G agree and that the L^2-Betti numbers of a lattice Gamma in G equal those of G up to scaling by the covolume of Gamma in G. We also deduce several vanishing results, including the vanishing of the reduced L^2-cohomology for amenable locally compact groups.

math.GR↗

arXiv2012

Amenability and vanishing of L^2-Betti numbers: an operator algebraic approach

We recast the Foelner condition in an operator algebraic setting and prove that it implies a certain dimension flatness property. Furthermore, it is proven that the Foelner condition generalizes the existing notions of amenability and that the enveloping von Neumann algebra arising from a Foelner algebra is automatically injective. As an application we show how our techniques unify the previously known results concerning vanishing of L^2-Betti numbers for amenable groups, groupoids and quantum groups and moreover provides a large class of new examples of algebras with vanishing L^2-Betti numbers.

math.OA↗

arXiv2011

Uniqueness of group-measure space Cartan subalgebras

These notes provide an account on four lectures given by Adrian Ioana at the Institut Henri Poincare in May 2011, regarding a number of "uniqueness of Cartan" type results obtained in a recent series of papers by Chifan-Peterson and Popa-Vaes. The text is purely expository.

math.OA↗

arXiv2011

A cohomological description of property (T) for quantum groups

We prove a Delorme-Guichardet type theorem for discrete quantum groups expressing property (T) of the quantum group in question in terms of its first cohomology groups. As an application, we show that the first L^2-Betti number of a discrete property (T) quantum group vanishes.

math.OA↗

arXiv2011

Applications of Foelner's condition to quantum groups

Using the Foelner condition for coamenable quantum groups we derive information about the ring theoretical structure of the Hopf algebras arising from such quantum groups, as well as an approximation result concerning the Murray von Neumann dimension associated with the corresponding the von Neumann algebra.

math.OA↗

arXiv2010

On the zeroth L^2-homology of a quantum group

We prove that the zeroth L^2-Betti number of a compact quantum group vanishes unless the underlying C*-algebra is finite dimensional and that the zeroth L^2-homology itself is non-trivial exactly when the quantum group is coamenable.

math.OA↗

arXiv2010

Property (T) and exotic quantum group norms

Utilizing the notion of property (T) we construct new examples of quantum group norms on the polynomial algebra of a compact quantum group, and provide criteria ensuring that these are not equal to neither the minimal nor the maximal norm. Along the way we generalize several classical operator algebraic characterizations of property (T) to the quantum group setting which unify recent approaches to property (T) for quantum groups with previous ones. The techniques developed furthermore provide tools to answer two open problems; firstly a question by Bédos, Murphy and Tuset about automatic continuity of the comultiplication and secondly a problem left open by Woronowicz regarding the structure of elements whose coproduct is a finite sum of simple tensors.

math.OA↗

arXiv2009

An L^2-Kunneth formula for tracial algebras

We prove a Kunneth formula computing the Connes-Shlyakhtenko L^2-Betti numbers of the algebraic tensor product of two tracial *-algebras in terms of the L^2-Betti numbers of the two original algebras. As an application, we construct examples of non-finite, non-cocommutative, compact quantum groups with a non-vanishing first L^2-Betti number.

math.OA↗

arXiv2008

L^2-Betti numbers of coamenable quantum groups

We prove that a compact quantum group is coamenable if and only if its corepresentation ring is amenable. We further propose a Foelner condition for compact quantum groups and prove it to be equivalent to coamenability. Using this Foelner condition, we prove that for a coamenable compact quantum group with tracial Haar state, the enveloping von Neumann algebra is dimension flat over the Hopf algebra of matrix coefficients. This generalizes a theorem of Lueck from the group case to the quantum group case, and provides examples of compact quantum groups with vanishing L^2-Betti numbers.

math.OA↗

arXiv2008

L^2-homology for compact quantum groups

A notion of L^2-homology for compact quantum groups is introduced, generalizing the classical notion for countable, discrete groups. If the compact quantum group in question has tracial Haar state, it is possible to define its L^2-Betti numbers and Novikov-Shubin invariants/capacities. It is proved that these L^2-Betti numbers vanish for the Gelfand dual of a compact Lie group and that the zeroth Novikov-Shubin invariant equals the dimension of the underlying Lie group. Finally, we relate our approach to the approach of A. Connes and D. Shlyakhtenko by proving that the L^2-Betti numbers of a compact quantum group, with tracial Haar state, are equal to the Connes-Shlyakhtenko L^2-Betti numbers of its Hopf *-algebra of matrix coefficients.

math.OA↗