239 research outputs found
Quasisplit Hecke algebras and symmetric spaces
Let (G,K) be a symmetric pair over an algebraically closed field of
characteristic different of 2 and let sigma be an automorphism with square 1 of
G preserving K. In this paper we consider the set of pairs (O,L) where O is a
sigma-stable K-orbit on the flag manifold of G and L is an irreducible
K-equivariant local system on O which is "fixed" by sigma. Given two such pairs
(O,L), (O',L'), with O' in the closure \bar O of O, the multiplicity space of
L' in the a cohomology sheaf of the intersection cohomology of \bar O with
coefficients in L (restricted to O') carries an involution induced by sigma and
we are interested in computing the dimensions of its +1 and -1 eigenspaces. We
show that this computation can be done in terms of a certain module structure
over a quasisplit Hecke algebra on a space spanned by the pairs (O,L) as above.Comment: 46 pages. Version 2 reorganizes the explicit calculation of the Hecke
module, includes details about computing \bar, and corrects small misprints.
Version 3 adds two pages relating this paper to unitary representation
theory, corrects misprints, and displays more equations. Version 4 corrects
misprints, and adds two cases previously neglected at the end of 7.
Parameters for Twisted Representations
The study of Hermitian forms on a real reductive group gives rise, in the
unequal rank case, to a new class of Kazhdan-Lusztig-Vogan polynomials. These
are associated with an outer automorphism of , and are related to
representations of the extended group . These polynomials were
defined geometrically by Lusztig and Vogan in "Quasisplit Hecke Algebras and
Symmetric Spaces", Duke Math. J. 163 (2014), 983--1034. In order to use their
results to compute the polynomials, one needs to describe explicitly the
extension of representations to the extended group. This paper analyzes these
extensions, and thereby gives a complete algorithm for computing the
polynomials. This algorithm is being implemented in the Atlas of Lie Groups and
Representations software
On elliptic factors in real endoscopic transfer I
This paper is concerned with the structure of packets of representations and
some refinements that are helpful in endoscopic transfer for real groups. It
includes results on the structure and transfer of packets of limits of discrete
series representations. It also reinterprets the Adams-Johnson transfer of
certain nontempered representations via spectral analogues of the
Langlands-Shelstad factors, thereby providing structure and transfer compatible
with the associated transfer of orbital integrals. The results come from two
simple tools introduced here. The first concerns a family of splittings of the
algebraic group G under consideration; such a splitting is based on a
fundamental maximal torus of G rather than a maximally split maximal torus. The
second concerns a family of Levi groups attached to the dual data of a
Langlands or an Arthur parameter for the group G. The introduced splittings
provide explicit realizations of these Levi groups. The tools also apply to
maps on stable conjugacy classes associated with the transfer of orbital
integrals. In particular, they allow for a simpler version of the definitions
of Kottwitz-Shelstad for twisted endoscopic transfer in certain critical cases.
The paper prepares for spectral factors in twisted endoscopic transfer that are
compatible in a certain sense with the standard factors discussed here. This
compatibility is needed for Arthur's global theory. The twisted factors
themselves will be defined in a separate paper.Comment: 48 pages, to appear in Progress in Mathematics, Volume 312,
Birkha\"user. Also renumbering to match that of submitted versio
Hecke algebras with unequal parameters and Vogan's left cell invariants
In 1979, Vogan introduced a generalised -invariant for characterising
primitive ideals in enveloping algebras. Via a known dictionary this translates
to an invariant of left cells in the sense of Kazhdan and Lusztig. Although it
is not a complete invariant, it is extremely useful in describing left cells.
Here, we propose a general framework for defining such invariants which also
applies to Hecke algebras with unequal parameters.Comment: 15 pages. arXiv admin note: substantial text overlap with
arXiv:1405.573
Unitary Dual of GL_n at archimedean places and global Jacquet-Langlands correspondence
In [7], results about the global Jacquet-Langlands correspondence, (weak and
strong) multiplicity-one theorems and the classification of automorphic
representations for inner forms of the general linear group over a number field
are established, under the condition that the local inner forms are split at
archimedean places. In this paper, we extend the main local results of [7] to
archimedean places so that this assumption can be removed. Along the way, we
collect several results about the unitary dual of general linear groups over
\bbR, \bbC or \bbH of independent interest
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
Dirac cohomology, elliptic representations and endoscopy
The first part (Sections 1-6) of this paper is a survey of some of the recent
developments in the theory of Dirac cohomology, especially the relationship of
Dirac cohomology with (g,K)-cohomology and nilpotent Lie algebra cohomology;
the second part (Sections 7-12) is devoted to understanding the unitary
elliptic representations and endoscopic transfer by using the techniques in
Dirac cohomology. A few problems and conjectures are proposed for further
investigations.Comment: This paper will appear in `Representations of Reductive Groups, in
Honor of 60th Birthday of David Vogan', edited by M. Nervins and P. Trapa,
published by Springe
Development of dielectric elastomer actuators for MRI devices
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references (leaves 100-104).Dielectric elastomer (DE) actuators are an emerging class of polymer actuation devices. They exhibit large strains and have high force and energy densities. They can be designed in a variety of geometries and are inexpensive to manufacture. Currently, the use of DE Actuation is limited because quantitative design information is incomplete and the complex phenomena governing their performance have not been fully characterized. In this study, several such issues are investigated both experimentally and analytically. The actuators designed for this research function as binary actuators, that is, they operate between two set states, OFF and ON. Performance of the actuators is predicted based on theoretical analysis and the results are compared to experimental results. Improvement of fabrication methods and determination of optimum design parameters have been experimentally determined. Since DE actuators can be constructed out of polymers and without any ferromagnetic materials, they can potentially be used in a Magnetic Resonance Imaging (MRI) machine, which has strict compatibility requirements that limit the use of certain materials. MRI is a powerful and effective medical diagnostic tool, but treatment is limited because of the confined space and compatibility issues. It has been well recognized that its value would be increased if it were possible to physically manipulate objects within the MRI machine during imaging, but conventional manipulation systems cannot operate within an MRI due to the incompatibility of ferromagnetic materials. Binary DE actuators eliminate the need for conventional electromagnetic actuators and their associated controlling electronics. This inherent compatibility suggests that a new class of MRI treatment devices(cont.) is possible. Potential applications for use in the MRI environment are introduced, and prototypes for illustrating these applications are fabricated. One such application, a reconfigurable RF coil for flexible imaging capabilities, proves that not only are DE actuators and MRI compatible, but that they can significantly enhance imaging capabilities.by John D. Vogan.S.M
Global analysis by hidden symmetry
Hidden symmetry of a G'-space X is defined by an extension of the G'-action
on X to that of a group G containing G' as a subgroup. In this setting, we
study the relationship between the three objects:
(A) global analysis on X by using representations of G (hidden symmetry);
(B) global analysis on X by using representations of G';
(C) branching laws of representations of G when restricted to the subgroup
G'.
We explain a trick which transfers results for finite-dimensional
representations in the compact setting to those for infinite-dimensional
representations in the noncompact setting when is -spherical.
Applications to branching problems of unitary representations, and to spectral
analysis on pseudo-Riemannian locally symmetric spaces are also discussed.Comment: Special volume in honor of Roger Howe on the occasion of his 70th
birthda
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