160 research outputs found
Star Products on Coadjoint Orbits
We study properties of a family of algebraic star products defined on
coadjoint orbits of semisimple Lie groups. We connect this description with the
point of view of differentiable deformations and geometric quantization.Comment: Talk given at the XXIII ICGTMP, Dubna (Russia) August 200
Notions of Infinity in Quantum Physics
In this article we will review some notions of infiniteness that appear in
Hilbert space operators and operator algebras. These include proper
infiniteness, Murray von Neumann's classification into type I and type III
factors and the class of F{/o} lner C*-algebras that capture some aspects of
amenability. We will also mention how these notions reappear in the description
of certain mathematical aspects of quantum mechanics, quantum field theory and
the theory of superselection sectors. We also show that the algebra of the
canonical anti-commutation relations (CAR-algebra) is in the class of F{/o}
lner C*-algebras.Comment: 11 page
Quantum twistors
We compute explicitly a star product on the Minkowski space whose Poisson
bracket is quadratic. This star product corresponds to a deformation of the
conformal spacetime, whose big cell is the Minkowski spacetime. The description
of Minkowski space is made in the twistor formalism and the quantization
follows by substituting the classical conformal group by a quantum group.Comment: 47 pages. references added, some parts rewritten. To appear in
'p-adic Numbers, Ultrametric Analysis and Applicarions
Torsion formulation of gravity
We make it precise what it means to have a connection with torsion as
solution of the Einstein equations. While locally the theory remains the same,
the new formulation allows for topologies that would have been excluded in the
standard formulation of gravity. In this formulation it is possible to couple
arbitrary torsion to gauge fields without breaking the gauge invariance.Comment: AMS-LaTeX, 25 pages. Appendices have been eliminated and the
necessary concepts have been inroduced in the text. We have added some
reference
Considerations on Super Poincare Algebras and their Extensions to Simple Superalgebras
We consider simple superalgebras which are a supersymmetric extension of
\fspin(s,t) in the cases where the number of odd generators does not exceed
64. All of them contain a super Poincar\'e algebra as a contraction and another
as a subalgebra. Because of the contraction property, some of these algebras
can be interpreted as de Sitter or anti de Sitter superalgebras. However, the
number of odd generators present in the contraction is not always minimal due
to the different splitting properties of the spinor representations under a
subalgebra. We consider the general case, with arbitrary dimension and
signature, and examine in detail particular examples with physical implications
in dimensions and .Comment: 16 pages, AMS-LaTeX. Version to appear in the Reviews in Mathematical
Physic
Twisted duality of the CAR-Algebra
We give a complete proof of the twisted duality property M(q)'= Z M(q^\perp)
Z* of the (self-dual) CAR-Algebra in any Fock representation. The proof is
based on the natural Halmos decomposition of the (reference) Hilbert space when
two suitable closed subspaces have been distinguished. We use modular theory
and techniques developed by Kato concerning pairs of projections in some
essential steps of the proof.
As a byproduct of the proof we obtain an explicit and simple formula for the
graph of the modular operator. This formula can be also applied to fermionic
free nets, hence giving a formula of the modular operator for any double cone.Comment: 32 pages, Latex2e, to appear in Journal of Mathematical Physic
A Comparative Analysis of 2D and 3D Tasks for Virtual Reality Therapies Based on Robotic-Assisted Neurorehabilitation for Post-stroke Patients
Post-stroke neurorehabilitation based on virtual therapies are performed completing repetitive exercises shown in visual electronic devices, whose content represents imaginary or daily life tasks. Currently, there are two ways of visualization of these task. 3D virtual environments are used to get a three dimensional space that represents the real world with a high level of detail, whose realism is determinated by the resolucion and fidelity of the objects of the task. Furthermore, 2D virtual environments are used to represent the tasks with a low degree of realism using techniques of bidimensional graphics. However, the type of visualization can influence the quality of perception of the task, affecting the patient's sensorimotor performance. The purpose of this paper was to evaluate if there were differences in patterns of kinematic movements when post-stroke patients performed a reach task viewing a virtual therapeutic game with two different type of visualization of virtual environment: 2D and 3D. Nine post-stroke patients have participated in the study receiving a virtual therapy assisted by PUPArm rehabilitation robot. Horizontal movements of the upper limb were performed to complete the aim of the tasks, which consist in reaching peripheral or perspective targets depending on the virtual environment shown. Various parameter types such as the maximum speed, reaction time, path length, or initial movement are analyzed from the data acquired objectively by the robotic device to evaluate the influence of the task visualization. At the end of the study, a usability survey was provided to each patient to analysis his/her satisfaction level. For all patients, the movement trajectories were enhanced when they completed the therapy. This fact suggests that patient's motor recovery was increased. Despite of the similarity in majority of the kinematic parameters, differences in reaction time and path length were higher using the 3D task. Regarding the success rates were very similar. In conclusion, the using of 2D environments in virtual therapy may be a more appropriate and comfortable way to perform tasks for upper limb rehabilitation of post-stroke patients, in terms of accuracy in order to effectuate optimal kinematic trajectories
Scherk-Schwarz Reduction of D=5 Special and Quaternionic Geometry
We give the N=2 gauged supergravity interpretation of a generic D=4, N=2
theory as it comes from generalized Scherk-Schwarz reduction of D=5, N=2
(ungauged) supergravity. We focus on the geometric aspects of the D=4 data such
as the general form of the scalar potential and masses in terms of the gauging
of a ``flat group''. Higgs and super-Higgs mechanism are discussed in some
detail.Comment: final version to be published on Class.Quant.Gra
On Central Charges and Hamiltonians for 0-brane dynamics
We consider general properties of central charges of zero branes and
associated duality invariants, in view of their double role, on the bulk and on
the world volume (quantum-mechanical) theory. A detailed study of the BPS
condition for the mass spectrum arising from toroidal compactifications is
given for 1/2, 1/4 and 1/8 BPS states in any dimensions. As a byproduct, we
retreive the U-duality invariant conditions on the charge (zero mode) spectrum
and the orbit classification of BPS states preserving different fractions of
supersymmetry. The BPS condition for 0-branes in theories with 16
supersymmetries in any dimension is also discussed.Comment: 23 pages, latex fil
Integration of massive states as contractions of non linear -models
We consider the contraction of some non linear sigma models which appear in
effective supergravity theories. In particular we consider the contractions of
maximally symmetric spaces corresponding to N=1 and N=2 theories, as they
appear in certain low energy effective supergravity actions with mass
deformations.
The contraction procedure is shown to describe the integrating out of massive
modes in the presence of interactions, as it happens in many supergravity
models after spontaneous supersymmetry breaking.Comment: AMS-LaTeX, 33 page
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