145 research outputs found
A Simple Proof of the Fundamental Theorem about Arveson Systems
With every Eo-semigroup (acting on the algebra of of bounded operators on a
separable infinite-dimensional Hilbert space) there is an associated Arveson
system. One of the most important results about Arveson systems is that every
Arveson system is the one associated with an Eo-semigroup. In these notes we
give a new proof of this result that is considerably simpler than the existing
ones and allows for a generalization to product systems of Hilbert module (to
be published elsewhere).Comment: Publication data added, acknowledgements and a note after acceptance
added, corrects a number of inconveniences that have been produced in the
published version during the publication proces
Representations of C*-dynamical systems implemented by Cuntz families
Given a dynamical system (A,\al) where is a unital \ca-algebra and
\al is a (possibly non-unital) *-endomorphism of , we examine families
such that is a representation of , is a
Toeplitz-Cuntz family and a covariance relation holds. We compute a variety of
non-selfadjoint operator algebras that depend on the choice of the covariance
relation, along with the smallest \ca-algebra they generate, namely the
\ca-envelope. We then relate each occurrence of the \ca-envelope to (a full
corner of) an appropriate twisted crossed product. We provide a counterexample
to show the extent of this variety. In the context of \ca-algebras, these
results can be interpreted as analogues of Stacey's famous result, for
non-automorphic systems and .
Our study involves also the one variable generalized crossed products of
Stacey and Exel. In particular, we refine a result that appears in the
pioneering paper of Exel on (what is now known as) Exel systems.Comment: 29 pages; changes in subsection 1.2; close to publicatio
The noncommutative Kubo Formula: Applications to Transport in Disordered Topological Insulators with and without Magnetic Fields
The non-commutative theory of charge transport in mesoscopic aperiodic
systems under magnetic fields, developed by Bellissard, Shulz-Baldes and
collaborators in the 90's, is complemented with a practical numerical
implementation. The scheme, which is developed within a -algebraic
framework, enable efficient evaluations of the non-commutative Kubo formula,
with errors that vanish exponentially fast in the thermodynamic limit.
Applications to a model of a 2-dimensional Quantum spin-Hall insulator are
given. The conductivity tensor is mapped as function of Fermi level, disorder
strength and temperature and the phase diagram in the plane of Fermi level and
disorder strength is quantitatively derived from the transport simulations.
Simulations at finite magnetic field strength are also presented.Comment: 10 figure
Scaling by 5 on a 1/4-Cantor Measure
Each Cantor measure (\mu) with scaling factor 1/(2n) has at least one
associated orthonormal basis of exponential functions (ONB) for L^2(\mu). In
the particular case where the scaling constant for the Cantor measure is 1/4
and two specific ONBs are selected for L^2(\mu), there is a unitary operator U
defined by mapping one ONB to the other. This paper focuses on the case in
which one ONB (\Gamma) is the original Jorgensen-Pedersen ONB for the Cantor
measure (\mu) and the other ONB is is 5\Gamma. The main theorem of the paper
states that the corresponding operator U is ergodic in the sense that only the
constant functions are fixed by U.Comment: 34 page
The Index of (White) Noises and their Product Systems
(See detailed abstract in the article.) We single out the correct class of
spatial product systems (and the spatial endomorphism semigroups with which the
product systems are associated) that allows the most far reaching analogy in
their classifiaction when compared with Arveson systems. The main differences
are that mere existence of a unit is not it sufficient: The unit must be
CENTRAL. And the tensor product under which the index is additive is not
available for product systems of Hilbert modules. It must be replaced by a new
product that even for Arveson systems need not coincide with the tensor
product
Algebraic approach to quantum field theory on non-globally-hyperbolic spacetimes
The mathematical formalism for linear quantum field theory on curved
spacetime depends in an essential way on the assumption of global
hyperbolicity. Physically, what lie at the foundation of any formalism for
quantization in curved spacetime are the canonical commutation relations,
imposed on the field operators evaluated at a global Cauchy surface. In the
algebraic formulation of linear quantum field theory, the canonical commutation
relations are restated in terms of a well-defined symplectic structure on the
space of smooth solutions, and the local field algebra is constructed as the
Weyl algebra associated to this symplectic vector space. When spacetime is not
globally hyperbolic, e.g. when it contains naked singularities or closed
timelike curves, a global Cauchy surface does not exist, and there is no
obvious way to formulate the canonical commutation relations, hence no obvious
way to construct the field algebra. In a paper submitted elsewhere, we report
on a generalization of the algebraic framework for quantum field theory to
arbitrary topological spaces which do not necessarily have a spacetime metric
defined on them at the outset. Taking this generalization as a starting point,
in this paper we give a prescription for constructing the field algebra of a
(massless or massive) Klein-Gordon field on an arbitrary background spacetime.
When spacetime is globally hyperbolic, the theory defined by our construction
coincides with the ordinary Klein-Gordon field theory on aComment: 21 pages, UCSBTH-92-4
The existence problem for dynamics of dissipative systems in quantum probability
Motivated by existence problems for dissipative systems arising naturally in
lattice models from quantum statistical mechanics, we consider the following
-algebraic setting: A given hermitian dissipative mapping is
densely defined in a unital -algebra . The identity
element in is also in the domain of . Completely
dissipative maps are defined by the requirement that the induced maps,
, are dissipative on the by complex
matrices over for all . We establish the existence of different
types of maximal extensions of completely dissipative maps. If the enveloping
von Neumann algebra of is injective, we show the existence of an
extension of which is the infinitesimal generator of a quantum
dynamical semigroup of completely positive maps in the von Neumann algebra. If
is a given well-behaved *-derivation, then we show that each of the
maps and is completely dissipative.Comment: 24 pages, LaTeX/REVTeX v. 4.0, submitted to J. Math. Phys.; PACS 02.,
02.10.Hh, 02.30.Tb, 03.65.-w, 05.30.-
Property (RD) for Hecke pairs
As the first step towards developing noncommutative geometry over Hecke
C*-algebras, we study property (RD) (Rapid Decay) for Hecke pairs. When the
subgroup H in a Hecke pair (G,H) is finite, we show that the Hecke pair (G,H)
has (RD) if and only if G has (RD). This provides us with a family of examples
of Hecke pairs with property (RD). We also adapt Paul Jolissant's works in 1989
to the setting of Hecke C*-algebras and show that when a Hecke pair (G,H) has
property (RD), the algebra of rapidly decreasing functions on the set of double
cosets is closed under holomorphic functional calculus of the associated
(reduced) Hecke C*-algebra. Hence they have the same K_0-groups.Comment: A short note added explaining other methods to prove that the
subalgebra of rapidly decreasing functions is smooth. This is the final
version as published. The published version is available at: springer.co
Involutive Categories and Monoids, with a GNS-correspondence
This paper develops the basics of the theory of involutive categories and
shows that such categories provide the natural setting in which to describe
involutive monoids. It is shown how categories of Eilenberg-Moore algebras of
involutive monads are involutive, with conjugation for modules and vector
spaces as special case. The core of the so-called Gelfand-Naimark-Segal (GNS)
construction is identified as a bijective correspondence between states on
involutive monoids and inner products. This correspondence exists in arbritrary
involutive categories
The Moyal bracket and the dispersionless limit of the KP hierarchy
A new Lax equation is introduced for the KP hierarchy which avoids the use of
pseudo-differential operators, as used in the Sato approach. This Lax equation
is closer to that used in the study of the dispersionless KP hierarchy, and is
obtained by replacing the Poisson bracket with the Moyal bracket. The
dispersionless limit, underwhich the Moyal bracket collapses to the Poisson
bracket, is particularly simple.Comment: 9 pages, LaTe
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