DOCSLIB.ORG

Parabolic Geometries I Background and General Theory

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Parabolic Geometries I Background and General Theory Mathematical Surveys and Monographs Volume 154 Parabolic Geometries I Background and General Theory Andreas Cˇ ap Jan Slovák American Mathematical Society http://dx.doi.org/10.1090/surv/154 Parabolic Geometries I Background and General Theory Mathematical Surveys and Monographs Volume 154 Parabolic Geometries I Background and General Theory Andreas Cˇ ap Jan Slovák American Mathematical Society Providence, Rhode Island EDITORIAL COMMITTEE Jerry L. Bona Michael G. Eastwood Ralph L. Cohen, Chair J. T. Stafford Benjamin Sudakov 2000 Mathematics Subject Classification. Primary 53C15, 53A40, 53B15, 53C05, 58A32; Secondary 53A55, 53C10, 53C30, 53D10, 58J70. The first author was supported during his work on this book at different times by projects P15747–N05 and P19500–N13 of the Fonds zur F¨orderung der wissenschaftlichen Forschung (FWF). The second author was supported during his work on this book by the grant MSM0021622409, “Mathematical structures and their physical applications” of the Czech Republic Research Intents scheme. Essential steps in the preparation of this book were made during the authors’ visit to the “Mathematisches Forschungsinstitut Oberwolfach” (MFO) in the framework of the Research in Pairs program. For additional information and updates on this book, visit www.ams.org/bookpages/surv-154 Library of Congress Cataloging-in-Publication Data Cap,ˇ Andreas, 1965– Parabolic geometries / Andreas Cap,ˇ Jan Slov´ak, v. cm. — (Mathematical surveys and monographs ; v. 154) Includes bibliographical references and index. Contents: 1. Background and general theory ISBN 978-0-8218-2681-2 (alk. paper) 1. Partial differential operators. 2. Conformal geometry. 3. Geometry, Projective. I. Slov´ak, Jan, 1960– II. Title. QA329.42.C37 2009 515.7242–dc22 2009009335 Copying and reprinting. Individual readers of this publication, and nonprofit libraries acting for them, are permitted to make fair use of the material, such as to copy a chapter for use in teaching or research. Permission is granted to quote brief passages from this publication in reviews, provided the customary acknowledgmentofthesourceisgiven. Republication, systematic copying, or multiple reproduction of any material in this publication is permitted only under license from the American Mathematical Society. Requests for such permission should be addressed to the Acquisitions Department, American Mathematical Society, 201 Charles Street, Providence, Rhode Island 02904-2294 USA. Requests can also be made by e-mail to [email protected]. c 2009 by the American Mathematical Society. All rights reserved. The American Mathematical Society retains all rights except those granted to the United States Government. Printed in the United States of America. ∞ The paper used in this book is acid-free and falls within the guidelines established to ensure permanence and durability. Visit the AMS home page at http://www.ams.org/ 10987654321 141312111009 Contents Preface vii Part 1. Background 1 Chapter 1. Cartan geometries 3 1.1. Prologue — a few examples of homogeneous spaces 4 1.2. Some background from differential geometry 15 1.3. A survey on connections 35 1.4. Geometry of homogeneous spaces 49 1.5. Cartan connections 70 1.6. Conformal Riemannian structures 112 Chapter 2. Semisimple Lie algebras and Lie groups 141 2.1. Basic structure theory of Lie algebras 141 2.2. Complex semisimple Lie algebras and their representations 160 2.3. Real semisimple Lie algebras and their representations 199 Part 2. General theory 231 Chapter 3. Parabolic geometries 233 3.1. Underlying structures and normalization 234 3.2. Structure theory and classification 290 3.3. Kostant’s version of the Bott–Borel–Weil theorem 339 Historical remarks and references for Chapter 3 360 Chapter 4. A panorama of examples 363 4.1. Structures corresponding to |1|–gradings 363 4.2. Parabolic contact structures 402 4.3. Examples of general parabolic geometries 426 4.4. Correspondence spaces and twistor spaces 455 4.5. Analogs of the Fefferman construction 478 Chapter 5. Distinguished connections and curves 497 5.1. Weyl structures and scales 498 5.2. Characterization of Weyl structures 517 5.3. Canonical curves 558 v vi CONTENTS Appendix A. Other prolongation procedures 599 Appendix B. Tables 607 Bibliography 617 Index 623 Preface The roots of the project to write this book originated in the early nineteen– nineties, when several streams of mathematical ideas met after being developed more or less separately for several decades. This resulted in an amazing interac- tion between active groups of mathematicians working in several directions. One of these directions was the study of conformally invariant differential operators re- lated, on the one hand, to Penrose’s twistor program (M.G. Eastwood, T.N. Bailey, R.J. Baston, C.R. Graham, A.R. Gover, and others) and, on the other hand, to hypercomplex analysis (V. Souˇcek, F. Sommen, and others). It turned out that, via the canonical Cartan connection or equivalent data, conformal geometry is just one instance of a much more general picture. Over the years we noticed that the foundations for this general picture had already been developed in the pioneering work of N. Tanaka. His work was set in the language of the equivalence problem and of differential systems, but independent of the much better disseminated de- velopments of the theory of differential systems linked to names like S.S. Chern, R. Bryant, and M. Kuranishi. While Tanaka’s work did not become widely known, it was further developed, in particular, by K. Yamaguchi and T. Morimoto, who put it in the setting of filtered manifolds and applied it to the geometric study of systems of PDE’s. A lot of input and stimulus also came from Ch. Fefferman’s work in complex analysis and geometric function theory, in particular, his parabolic invariant theory program and the relation between CR–structures and conformal structures. Finally, via the homogeneous models, all of these studies have close relations to various parts of representation theory of semisimple Lie groups and Lie algebras, developed for example by T. Branson and B. Ørsted. Enjoying the opportunities offered by the newly emerging International Erwin Schr¨odinger Institute for Mathematical Physics (ESI) in Vienna as well as the long lasting tradition of the international Winter Schools “Geometry and Physics” held every year in Srn´ı, Czech Republic, the authors of this book started a long and fruitful collaboration with most of the above mentioned people. Step by step, all of the general concepts and problems were traced back to old masters like Schouten, Veblen, Thomas, and Cartan, and a broad research program led to a conceptual understanding of the common background of the various approaches developed more recently. In the late nineties, the general version of the invariant calculus for a vast class of geometrical structures extended the tools for geometric analy- sis on homogeneous vector bundles and the direct applications of representation theory expanded in this way to situations involving curvatures. In particular, the celebrated Bernstein–Gelfand–Gelfand resolutions were recovered in the realm of general parabolic geometries by V. Souˇcek and the authors and the cohomological substance of all these constructions was clarified. vii viii PREFACE The book was written with two goals in mind. On one hand, we want to provide a (relatively) gentle introduction into this fascinating world which blends algebra and geometry, Lie theory and geometric analysis, geometric intuition and categorical thinking. At the same time, preparing the first treatment of the general theory and collection of the main results on the subject, gave us the ambition to provide the standard reference for the experts in the area. These two goals are reflected in the overall structure of the book which finally developed into two volumes. The second volume will be called “Parabolic Geometries II: Invariant Differential Operators and Applications”. Contents of the book. The basic theme of this book is canonical Cartan connections associated to certain types of geometric structures, which immediately causes peculiarities. Cartan connections and, more generally, various types of absolute parallelisms certainly played a central role in Cartan’s work on differential geometry and they still belong to the basic tools in several geometric approaches to differential equa- tions. However, in the efforts in the second half of the last century to put Cartan’s ideas into the conceptual framework of fiber bundles, the main part was taken over by principal connections. The isolated treatments in the books by Kobayashi and Sharpe stopped at alternative presentations of the quite well–known conformal Rie- mannian and projective structures. As a consequence, even basic facts on Cartan connections are neither well represented in the standard literature on differential geometry nor well known among many people working in the field. For this book, we have collected some general facts about Cartan connections on principal fiber bundles in Section 1.5. While this is located in the “Background” part of the book because of its nature, quite a bit of this material will probably be new to most readers. The other peculiar consequence of the approach is that the geometric structures we study display strong similarities in the picture of Cartan connections, while they are extremely diverse in their original descriptions. Therefore, in large parts of the book we will take the point of view that the Cartan picture is the “true” description of the structures in question, while the original description is obtained as an underlying structure. This point of view is justified by the results on equivalences of categories between Cartan geometries and underlying structures in Section 3.1, which are among the main goals of this volume. This point of view will also be taken in the second volume, in which the Cartan connection (or some equivalent data) will be simply considered as an input. Let us describe the contents of the first volume in more detail. The techni- cal core of the book is Chapter 3.
Load more

Recommended publications
  • UTM Naive Lie Theory (John Stillwell) 0387782141.Pdf
    Undergraduate Texts in Mathematics Editors S. Axler K.A. Ribet Undergraduate Texts in Mathematics Abbott: Understanding Analysis. Daepp/Gorkin: Reading, Writing, and Proving: Anglin: Mathematics: A Concise History and A Closer Look at Mathematics. Philosophy. Devlin: The Joy of Sets: Fundamentals Readings in Mathematics. of-Contemporary Set Theory. Second edition. Anglin/Lambek: The Heritage of Thales. Dixmier: General Topology. Readings in Mathematics. Driver: Why Math? Apostol: Introduction to Analytic Number Theory. Ebbinghaus/Flum/Thomas: Mathematical Logic. Second edition. Second edition. Armstrong: Basic Topology. Edgar: Measure, Topology, and Fractal Geometry. Armstrong: Groups and Symmetry. Second edition. Axler: Linear Algebra Done Right. Second edition. Elaydi: An Introduction to Difference Equations. Beardon: Limits: A New Approach to Real Third edition. Analysis. Erdos/Sur˜ anyi:´ Topics in the Theory of Numbers. Bak/Newman: Complex Analysis. Second edition. Estep: Practical Analysis on One Variable. Banchoff/Wermer: Linear Algebra Through Exner: An Accompaniment to Higher Mathematics. Geometry. Second edition. Exner: Inside Calculus. Beck/Robins: Computing the Continuous Fine/Rosenberger: The Fundamental Theory Discretely of Algebra. Berberian: A First Course in Real Analysis. Fischer: Intermediate Real Analysis. Bix: Conics and Cubics: A Concrete Introduction to Flanigan/Kazdan: Calculus Two: Linear and Algebraic Curves. Second edition. Nonlinear Functions. Second edition. Bremaud:` An Introduction to Probabilistic Fleming: Functions of Several Variables. Second Modeling. edition. Bressoud: Factorization and Primality Testing. Foulds: Combinatorial Optimization for Bressoud: Second Year Calculus. Undergraduates. Readings in Mathematics. Foulds: Optimization Techniques: An Introduction. Brickman: Mathematical Introduction to Linear Franklin: Methods of Mathematical Programming and Game Theory. Economics. Browder: Mathematical Analysis: An Introduction. Frazier: An Introduction to Wavelets Through Buchmann: Introduction to Cryptography.
    [Show full text]
  • 132 Lie's Theory of Continuous Groups Plays Such an Important Part in The
    132 SHORTER NOTICES. [Dec, Lie's theory of continuous groups plays such an important part in the elementary problems of the theory of differential equations, and has proved to be such a powerful weapon in the hands of competent mathematicians, that a work on differential equations, of even the most modest scope, appears decidedly incomplete without some account of it. It is to be regretted that Mr. Forsyth has not introduced this theory into his treatise. The introduction of a brief account of Runge's method for numerical integration is a very valuable addition to the third edition. The treatment of the Riccati equation has been modi­ fied. The theorem that the cross ratio of any four solutions is constant is demonstrated but not explicitly enunciated, which is much to be regretted. From the point of view of the geo­ metric applications, this is the most important property of the equations of the Riccati type. The theory of total differential equations has been discussed more fully than before, and the treatment on the whole is lucid. The same may be said of the modified treatment adopted by the author for the linear partial differential equations of the first order. A very valuable feature of the book is the list of examples. E. J. WlLCZYNSKI. Introduction to Projective Geometry and Its Applications. By ARNOLD EMOH. New York, John Wiley & Sons, 1905. vii + 267 pp. To some persons the term projective geometry has come to stand only for that pure science of non-metric relations in space which was founded by von Staudt.
    [Show full text]
  • Introduction to Representation Theory by Pavel Etingof, Oleg Golberg
    Introduction to representation theory by Pavel Etingof, Oleg Golberg, Sebastian Hensel, Tiankai Liu, Alex Schwendner, Dmitry Vaintrob, and Elena Yudovina with historical interludes by Slava Gerovitch Licensed to AMS. License or copyright restrictions may apply to redistribution; see http://www.ams.org/publications/ebooks/terms Licensed to AMS. License or copyright restrictions may apply to redistribution; see http://www.ams.org/publications/ebooks/terms Contents Chapter 1. Introduction 1 Chapter 2. Basic notions of representation theory 5 x2.1. What is representation theory? 5 x2.2. Algebras 8 x2.3. Representations 9 x2.4. Ideals 15 x2.5. Quotients 15 x2.6. Algebras defined by generators and relations 16 x2.7. Examples of algebras 17 x2.8. Quivers 19 x2.9. Lie algebras 22 x2.10. Historical interlude: Sophus Lie's trials and transformations 26 x2.11. Tensor products 30 x2.12. The tensor algebra 35 x2.13. Hilbert's third problem 36 x2.14. Tensor products and duals of representations of Lie algebras 36 x2.15. Representations of sl(2) 37 iii Licensed to AMS. License or copyright restrictions may apply to redistribution; see http://www.ams.org/publications/ebooks/terms iv Contents x2.16. Problems on Lie algebras 39 Chapter 3. General results of representation theory 41 x3.1. Subrepresentations in semisimple representations 41 x3.2. The density theorem 43 x3.3. Representations of direct sums of matrix algebras 44 x3.4. Filtrations 45 x3.5. Finite dimensional algebras 46 x3.6. Characters of representations 48 x3.7. The Jordan-H¨oldertheorem 50 x3.8. The Krull-Schmidt theorem 51 x3.9.
    [Show full text]
  • Lie Groups Notes
    Notes for Lie Theory Ting Gong Started Feb. 7, 2020. Last revision March 23, 2020 Contents 1 Matrix Analysis 1 1.1 Matrix groups . .1 1.2 Topological Properties . .4 1.3 Lie groups and homomorphisms . .6 1.4 Matrix Exponential . .8 1.5 Matrix Logarithm . .9 1.6 Polar Decomposition and More . 11 2 Lie Algebra and Representation 14 2.1 Definitions . 14 2.2 Lie algebra of Matrix Group . 16 2.3 Mixed Topics: Homomorphisms and Complexification . 18 i Chapter 1 Matrix Analysis 1.1 Matrix groups Last semester, in our reading group on differential topology, we analyzed some common Lie groups by treating them as an embedded submanifold. And we calculated the dimension by implicit function theorem. Now we discuss an algebraic approach to Lie groups. We assume familiarity in group theory, and we consider Lie groups as topological groups, namely, putting a topology on groups. This is going to be used in matrix analysis. Definition 1.1.1. Let M(n; k) be the set of n × n matrices, where k is a field (such as C; R). Let GL(n; k) ⊂ M(n; k) be the set of n × n invertible matrices with entries in k. Then we call this the general linear group over k. n2 Remark 1.1.2. Now we let k = C, and we can consider M(n; C) as C with the Euclidean topology. Thus we can define basic terms in analysis. Definition 1.1.3. Let fAmg ⊂ M(n; C) be a sequence. Then Am converges to A if each entry converges to the cooresponding entry in A in Euclidean metric space.
    [Show full text]
  • Sophus Lie: a Real Giant in Mathematics by Lizhen Ji*
    Sophus Lie: A Real Giant in Mathematics by Lizhen Ji* Abstract. This article presents a brief introduction other. If we treat discrete or finite groups as spe- to the life and work of Sophus Lie, in particular cial (or degenerate, zero-dimensional) Lie groups, his early fruitful interaction and later conflict with then almost every subject in mathematics uses Lie Felix Klein in connection with the Erlangen program, groups. As H. Poincaré told Lie [25] in October 1882, his productive writing collaboration with Friedrich “all of mathematics is a matter of groups.” It is Engel, and the publication and editing of his collected clear that the importance of groups comes from works. their actions. For a list of topics of group actions, see [17]. Lie theory was the creation of Sophus Lie, and Lie Contents is most famous for it. But Lie’s work is broader than this. What else did Lie achieve besides his work in the 1 Introduction ..................... 66 Lie theory? This might not be so well known. The dif- 2 Some General Comments on Lie and His Impact 67 ferential geometer S. S. Chern wrote in 1992 that “Lie 3 A Glimpse of Lie’s Early Academic Life .... 68 was a great mathematician even without Lie groups” 4 A Mature Lie and His Collaboration with Engel 69 [7]. What did and can Chern mean? We will attempt 5 Lie’s Breakdown and a Final Major Result . 71 to give a summary of some major contributions of 6 An Overview of Lie’s Major Works ....... 72 Lie in §6.
    [Show full text]
  • Very Basic Lie Theory
    VERY BASIC LIE THEORY ROGER HOWE Department of Mathematics, Yale University, New Haven, CT 06520 Lie theory, the theory of Lie groups, Lie algebras and their applications, is a fundamental part of mathematics. Since World War II it has been the focus of a burgeoning research effort, and is now seen to touch a tremendous spectrum of mathematical areas, including classical, differential, and algebraic geometry, topology, ordinary and partial differential equations, complex analysis (one and several variables), group and ring theory, number theory, and physics, from classical to quantum and relativistic. It is impossible in a short space to convey the full compass of the subject, but we will cite some examples. An early major success of Lie theory, occurring when the subject was still in its infancy, was to provide a systematic understanding of the relationship between Euclidean geometry and the newer geometries (hyperbolic non-Euclidean or Lobachevskian, Riemann's elliptic geometry, and projective geometry) that had arisen in the 19th century. This led Felix Klein to enunciate his Erlanger Programm [Kl] for the systematic understanding of geometry. The principle of Klein's program was that geometry should be understood as the study of quantities left invariant by the action of a group on a space. Another development in which Klein was involved was the Uniformization Theorem [Be] for Riemann surfaces. This theorem may be understood as saying that every connected two-manifold is a double coset space of the isometry group of one of the 3 (Euclidean, hyperbolic, elliptic) standard 2-dimensional geometries. (See also the recent article [F] in this MONTHLY.) Three-manifolds are much more complex than two manifolds, but the intriguing work of Thurston [Th] hi!s gone a long way toward showing that much of their structure can be understood in a way analogous to the 2-dimensional situation in terms of coset spaces of certain Lie groups.
    [Show full text]
  • Matrix Groups and Their Lie Algebras
    MATRIX GROUPS AND THEIR LIE ALGEBRAS ELIJAH SORIA Faculty Advisor: Dr. Keely Machmer-Wessels Saint Mary's College Mathematics, 2016 Abstract. The purpose of this paper is to provide an introduction to Lie Theory through the use of matrix groups and examples of Lie groups and Lie algebras that pertain to matrix groups. We begin by giving background information in the mathematical areas that are used in the study of Lie groups and Lie algebras, which are mainly abstract algebra, topology, and linear algebra. Second, we introduce the reader to a formal definition of a matrix group, as well as give three examples; the general linear group, the special linear group, and the orthogonal group. Next, we define the meaning of a Lie algebra and how Lie algebras are developed through the use of matrix exponentiation, and then use this method in finding the Lie algebras of the three examples of matrix groups previously mentioned. Last, we prove the result that all matrix groups are Lie groups, and then conclude the essay with a small discussion on the importance of Lie groups and Lie algebras. Date: May 15, 2016. 0 MATRIX GROUPS AND THEIR LIE ALGEBRAS 1 1. Introduction The origins of Lie theory stem from the work of Felix Klein (1849-1925), who envisioned that the geometry of space is determined by the group of its symmetries. The developments in modern physics in the 19th century required an expansion in our understanding of geometry in space, and thus the notions of Lie groups and their representations expanded as well.
    [Show full text]
  • Interaction Between Lie Theory and Algebraic Geometry
    INTERACTIONS BETWEEN LIE THEORY AND ALGEBRAIC GEOMETRY SHILIN YU 1. Introduction My primary research interests lie in the interactions of complex/algebraic geometry with Lie the- ory and representation theory in the spirit of noncommutative geometry, derived algebraic geometry and mathematical physics. Both Lie theory and algebraic geometry have been at the center of the 20th-century mathematical studies. Entering the 21th century, the two subjects came across each other in a surprising way, due to the phenomenal work of Kontsevich on deformation quantization of Poisson manifolds ([45]) and the birth of derived algebraic geometry ([46]). We start from the following observation: in algebraic geometry, the well-known Grothendieck- Riemann-Roch (GRR) theorem involves the Todd class, which is a characteristic class defined by a certain formal power series. The same type of class also appears in the Atiyah-Singer index theorem, which is a generalization of the Riemann-Roch theorem and one of the most important theorems of the 20th century. On the other hand, the exact same power series also shows up in Lie theory in a fundamental way, long before the GRR theorem was formulated. Namely, it appears in in the differential of the exponential map of a Lie group the Baker-Campbell-Hausdorff (BCH) formula in Lie theory. It (or its inverse) also plays an essential role in the Duflo-Kirillov isomorphism ([28]). Due to the work of many people, including but not limited to Kapranov [40], Kontsevich [45], Markarian [48], and Ramadoss [53], it is now clear that the same power series appearing in the two seemly unrelated cases is not a coincidence.
    [Show full text]
  • Historical Review of Lie Theory 1. the Theory of Lie
    Historical review of Lie Theory 1. The theory of Lie groups and their representations is a vast subject (Bourbaki [Bou] has so far written 9 chapters and 1,200 pages) with an extraordinary range of applications. Some of the greatest mathematicians and physicists of our times have created the tools of the subject that we all use. In this review I shall discuss briefly the modern development of the subject from its historical beginnings in the mid nineteenth century. The origins of Lie theory are geometric and stem from the view of Felix Klein (1849– 1925) that geometry of space is determined by the group of its symmetries. As the notion of space and its geometry evolved from Euclid, Riemann, and Grothendieck to the su- persymmetric world of the physicists, the notions of Lie groups and their representations also expanded correspondingly. The most interesting groups are the semi simple ones, and for them the questions have remained the same throughout this long evolution: what is their structure, where do they act, and what are their representations? 2. The algebraic story: simple Lie algebras and their representations. It was Sopus Lie (1842–1899) who started investigating all possible (local)group actions on manifolds. Lie’s seminal idea was to look at the action infinitesimally. If the local action is by R, it gives rise to a vector field on the manifold which integrates to capture the action of the local group. In the general case we get a Lie algebra of vector fields, which enables us to reconstruct the local group action.
    [Show full text]
  • Combinatorial Models for Lie Algebra Representations Rob Donnelly April 14, 2006
    Combinatorial models for Lie algebra representations Rob Donnelly April 14, 2006 Boolean lattices, revisited: n o Let Bn := subsets of {1, 2, . , n} . We think of Bn as a partially ordered set with respect to subset containment “⊆,” so we have S ⊆ T for S, T ∈ Bn iff S is a subset of T when we think of S and T as subsets of {1, 2, . , n}. For S and T in Bn, write S → T iff S ⊆ T and |T \ S| = 1. These edges are depicted in the order diagram for B3 below: {1, 2, 3} @ r@ @ @ @ {1, 2}{ 1, 3}{@ 2, 3} @ @ @ r@ rr @ @ @ @ @ @ {1}{ @ 2}{@ 3} @ @ r rr @ @ @ @ ∅ r The “Boolean Lattice” B3 All of the edges are taken to be pointing “up.” Let V = V [B ] = span {v S ∈ B }, a 2n-dimensional complex vector space. The subalgebra of gl(V ) n C S n generated by the following linear transformations X, Y , and H on V is isomorphic to g(A1): X X(vS) := vT T ∈Bn,S→T X Y (vS) := vR R∈Bn,R→S H(vS) := (2|S| − n) vS ↑ ↑ rank length So in our example above, X(v{2}) = v{1,2} + v{2,3}, Y (v{2}) = v∅, and H(v{2}) = (2 · 1 − 3)v{2} = −v{2}. Question: Given any complex finite-dimensional representation of a complex finite-dimensional semisimple Lie algebra, is it possible to find a similar kind of combinatorial model? 1 Answer: Yes. Here’s how. Let g := g(Γ,A) for a GCM graph (Γ,A) on n nodes so that g is finite-dimensional.
    [Show full text]
  • Matrix Lie Groups and Control Theory
    MATRIX LIE GROUPS AND CONTROL THEORY Jimmie Lawson Summer, 2007 Chapter 1 Introduction Mathematical control theory is the area of application-oriented mathematics that treats the basic mathematical principles, theory, and problems underly- ing the analysis and design of control systems, principally those encountered in engineering. To control an object means to influence its behavior so as to achieve a desired goal. One major branch of control theory is optimization. One assumes that a good model of the control system is available and seeks to optimize its behavior in some sense. Another major branch treats control systems for which there is uncertain- ity about the model or its environment. The central tool is the use of feedback in order to correct deviations from and stabilize the desired behavior. Control theory has its roots in the classical calculus of variations, but came into its own with the advent of efforts to control and regulate machinery and to develop steering systems for ships and much later for planes, rockets, and satellites. During the 1930s, researchers at Bell Telephone Laboratories developed feedback amplifiers, motiviated by the goal of assuring stability and appropriate response for electrical circuits. During the Second World War various military implementations and applications of control theory were developed. The rise of computers led to the implementation of controllers in the chemical and petroleum industries. In the 1950s control theory blossomed into a major field of study in both engineering and mathematics, and powerful techniques were developed for treating general multivariable, time-varying systems. Control theory has continued to advance with advancing technology, and has emerged in modern times as a highly developed discipline.
    [Show full text]
  • Algebraic Lie Theory
    Isaac Newton Institute for Mathematical Sciences Algebraic Lie Theory 12 January to 26 June 2009 Organisers: Professor M Geck (Aberdeen), Professor A Kleshchev (Oregon), Professor G Röhrle (Ruhr-Universität Bochum) A central theme in mathematics is to study blocks for all representations - of the various symmetries: an object or a structure is structures arising in Algebraic Lie Theory. symmetrical if it looks the same after a specific type of change is applied to it. This It is remarkable that a number of such very general concept occurs in a huge variety problems can be stated in relatively of situations, ranging from classical geometry elementary terms, but their solution (if at all to natural phenomena in physics and achieved!) may require a highly complex chemistry, e.g., crystal structures. Perhaps chain of theoretical arguments, or a the most familiar symmetrical objects are the complicated large-scale computer-aided five Platonic solids, or regular polyhedra. calculation. An example is given by the Mathematicians study such symmetries, and recent determination of the irreducible their higher-dimensional analogues, by representations of Lie groups of “type E8 ” investigating the correspondi ng “symmetry by a team of 18 mathematicians and group”, that is, the possible transformations computer scientists from the U.S. and of the object which preserve the symmetry. Europe. (The figure, taken from D. A. Vogan’s MIT talk on this event, shows the The programme is named after Sophus Lie root system of E8.) It is now clear that the (pronounced “lee”), a Norwegian solution of the open problems will only be mathematician who lived in the 19th century.
    [Show full text]