New Developments in Lie Theory and Geometry: 6th Workshop on Lie Theory and Geometry November 13-17, 2007 Cruz Chica, Cordoba, Argentina

363 Pages·2009·2.05 MB·English

by  Carolyn S. Gordon

C
ONTEMPORARY
M
ATHEMATICS
491
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American Mathematical Society
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New Developments in Lie
Theory and Geometry
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C
ONTEMPORARY
M
ATHEMATICS
491
New Developments in Lie
Theory and Geometry
Sixth Workshop on Lie Theory and Geometry
November 13-17, 2007
Cruz Chica, Córdoba, Argentina
Carolyn S. Gordon
Juan Tirao
Jorge A. Vargas
Joseph A. Wolf
Editors
American Mathematical Society
Providence, Rhode Island
Editorial Board
Dennis DeTurck, managing editor
George Andrews Abel Klein Martin J. Strauss
2000 Mathematics Subject Classification. Primary22Exx, 43A85, 53Cxx, 53C25, 53C30,
20Gxx, 58J53.
Library of Congress Cataloging-in-Publication Data
Workshop on Lie Theory and Geometry (6th : 2007 : Cruz Chica, La Cumbre, C´ordoba, Ar-
gentina)
Newdevelopmentsinlietheoryandgeometry/CarolynS.Gordon...[etal.],editors.
p.cm. —(Contemporarymathematics;v.491)
Includesbibliographicalreferences.
ISBN978-0-8218-4651-3(alk.paper)
1. Representations of Lie groups—Congresses. 2. Homogeneous spaces—Congresses.
3.Geometry,Differential—Congresses. I.Gordon,CarolynS.(CarolynSue),1950– II.Title.
QA387.W67 2007
512(cid:1).482—dc22 2009007622
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Contents
Preface vii
Einstein solvmanifolds and nilsolitons
Jorge Lauret 1
Algebraic sets associated to isoparametric submanifolds
Cristia´n U. Sa´nchez 37
Mostow strong rigidity and nonisomorphism for outer automorphism groups of
free groups and mapping class groups
Lizhen Ji 57
Spectral properties of flat manifolds
Roberto J. Miatello and Juan Pablo Rossetti 83
Heat content, heat trace, and isospectrality
P. Gilkey 115
LR-algebras
Dietrich Burde, Karel Dekimpe, and Sandra Deschamps 125
Combinatorial properties of generalized binomial coefficients
Chal Benson and Gail Ratcliff 141
Spherical functions for the action of a finite unitary group on a finite
Heisenberg group
Chal Benson and Gail Ratcliff 151
Application of the Weil representation: diagonalization of the discrete Fourier
transform
Shamgar Gurevich and Ronny Hadani 167
Infinite dimensional multiplicity free spaces II: Limits of commutative
nilmanifolds
Joseph A. Wolf 179
Certain components of Springer fibers: algorithms, examples and applications
L. Barchini and R. Zierau 209
Weighted Vogan diagrams associated to real nilpotent orbits
Esther Galina 239
The Gelfand-Zeitlin integrable system and its action on generic elements of
gl(n) and so(n)
Mark Colarusso 255
v
vi CONTENTS
Closed orbits of semisimple group actions and the real Hilbert-Mumford
function
Patrick Eberlein and Michael Jablonski 283
New techniques for pointed Hopf algebras
Nicola´s Andruskiewitsch and Fernando Fantino 323
Preface
Lie theory and differential geometry play tightly intertwined roles in many ac-
tive areas of mathematics and physics. The Sixth Workshop on Lie Theory and
Geometry was held in the province of Co´rdoba, Argentina November 13-17, 2007.
Theworkshopwasprecededby aoneday Conference on Lie Theory and Geometry
in honor of the sixtieth birthdays of Isabel Dotti and Roberto Miatello, who are
among those persons that have devoted themselves to developing the Universidad
NacionaldeC´ordobaasamajorcenterforLietheory. Whiletheearliestworkshops
intheserieswere focusedentirelyonthetheoryofLiegroupsandtheirrepresenta-
tions, later workshops began moving more towards applications of Lie theory. The
sixthworkshopfullyrealizedthisexpansion,withconsiderablefocusonapplications
of Lie groups in geometry, while still continuing to address representation theory.
Theworkshopemphasizedthreemajortopicsandtheirinteractions: representation
theory, geometric structures (in particular, homogeneous spaces), and applications
of Lie groups to spectral geometry. The diversity of themes was quite effective.
Many of the lectures on representation theory had a geometric bent while many of
themoregeometriclecturesappliedresultsfromrepresentationtheory. The result-
ing high level of interaction among researchers served to broaden the perspectives
of all the participants. The goal of this volume is to bring to a greater audience
not only the many interesting presentations but, more importantly, the bridging of
ideas.
Jorge Lauret presents a comprehensive exposition on left-invariant Einstein
metrics on noncompact Lie groups, an area that has seen significant advances in
thepastfewyears. AstrikingconnectionbetweenEinsteinmetricsonsolvmanifolds
and Ricci soliton metrics on nilmanifolds allows existence and classification ques-
tions for both types of structures to be addressed simultaneously. The article not
only surveys extensive results but also explains the techniques involved, including
geometric invariant theory.
Cristia´n Sa´nchez studies isoparametric submanifolds by introducing an alge-
braic structure on their planar normal sections. Among the results is a new char-
acterization of Cartan’s isoparametric hypersurfaces. An important setting here
is that of extrinsically homogeneous submanifolds in which the isometries of the
ambient space that preserve the submanifold act transitively on the submanifold.
An exposition by Lizhen Ji connects discrete subgroups of Lie groups with
related discrete groups in the broad context of geometric group theory. Analogs
of Mostow strong rigidity are discussed in the various settings. The exposition is
accessible to non-experts in geometric group theory.
Inversespectralgeometryaskstheextenttowhichspectraldataassociatedwith
a Riemannian manifold encode the geometry of the manifold. Juan Pablo Rossetti
vii
viii PREFACE
andRobertoMiatellopresent techniquesforconstructingisospectral flat manifolds
and survey a vast array of examples. The setting of flat manifolds makes the
material readily accessible to readers who are not familiar with spectral geometry,
while the rich behavior of the examples will also interest experts in the area. Peter
Gilkeyaddressesthetotalheatcontentfunctionandheattracealongwithquestions
of isospectrality for the Laplacian with Dirichlet boundary conditions on compact
manifoldswithboundary. Whiletheheattraceanditsasymptoticsprovideawidely
usedsource of spectralinvariants, itisnot knownwhetherthetotalheat content is
a spectral invariant. Gilkey’s article contains an exposition of the various concepts
along with new results, examples and open questions.
The group of NIL-affine transformations of a real simply-connected nilpotent
Lie group N is the semi-direct product of the translations and automorphisms of
N. Recently, Dietrich Burde, Karel Dekimpe and Sandra Deschamps showed that
N admits a simply transitive NIL-affine action of Rn if and only if the Lie algebra
of N adimits a complete “LR-structure”. In their article, Burde, Dekimpe and
Deschamps explain the notion of LR-structure , address questions of existence and
classification, and begin a study of their structure.
CommutativespacesarehomogeneousspacesG/K whereGislocallycompact,
K compact,andtheconvolutionalgebraL1(K\G/K)ofintegrablebi–K–invariant
functions is commutative; one also says that (G,K) is a Gelfand pair. Most com-
mutative spaces are weakly symmetric spaces, and analysis on them is amenable
to many techniques of classical analysis. Four articles address aspects of repre-
sentation theory tied to commutative nilmanifolds, that is, commutative spaces
G/K on which a nilpotent subgroup of G acts transitively. Chal Benson and Gail
Ratcliffstudycombinatorialpropertiesofthegeneralizedbinomialcoefficientsthat
they use to construct spherical functions for real Gelfand pairs (G,K) where K is
compact and G is the semidirect product H (cid:1)K of K with a Heisenberg group.
In a second article they replace the reals by any finite field of odd characteristic
but restrict K to the corresponding unitary group. Shamgar Gurevich and Ronny
Hadani introduce a new formulation for construction of the Weil representation in
thefinitefieldsetting,usingthefiniteHeisenberggroupandallpolarizationsrather
than making a (necessarily non–invariant) choice of polarization, and they use it
to study a particular diagonalization of the discrete Fourier transform. Finally,
Joseph Wolf studies direct limits of Gelfand pairs (G ,K ), G =N (cid:1)K with N
i i i i i i
nilpotent, and shows that the limit retains certain of the Gelfand pair properties,
in particular the multiplicity free property of the regular representation.
Three papers address aspects of the representation theory of real reductive
Lie groups. The associated cycle of an admissible representation is a construction
thatgivesgeometricinformationontherepresentation. LeticiaBarchiniandRoger
Zierau show how to compute the associated cycles for parabolically induced repre-
sentations of real general linear groups and for discrete series representations of a
numberofotherclassicalrealreductiveLiegroups. EstherGalinagoesfurtherinto
thestructureofK–orbitsinthenilpotentsetandstudiesweightedVogandiagrams
in conjunction with the classification of distinguished parabolic subalgebras. Mark
ColarussodescribescertainresultsofKostantandWallachfortheLiealgebrasgl(n)
and shows how they hold as well for orthogonal algebras so(n).