An Introduction to Quiver Representations

346 Pages·2017·1.731 MB·English

by  Harm Derksen;Jerzy Weyman

GRADUATE STUDIES 184
IN MATHEMATICS
An Introduction
to Quiver
Representations
Harm Derksen
Jerzy Weyman
American Mathematical Society
An Introduction
to Quiver
Representations

GRADUATE STUDIES 184
IN MATHEMATICS
An Introduction
to Quiver
Representations
Harm Derksen
Jerzy Weyman
American Mathematical Society
Providence, Rhode Island
EDITORIAL COMMITTEE
Dan Abramovich
Daniel S. Freed (Chair)
Gigliola Staffilani
Jeff A. Viaclovsky
2010 Mathematics Subject Classification. Primary 16G20, 16G10,16G70, 14L24, 13A50.
For additional informationand updates on this book, visit
www.ams.org/bookpages/gsm-184
Library of Congress Cataloging-in-Publication Data
Derksen,Harm,1970-|Weyman,Jerzy,1955-
Title: Anintroductiontoquiverrepresentations/HarmDerksen,JerzyWeyman.
Description: Providence,RhodeIsland: AmericanMathematicalSociety,[2017]|Series: Gradu-
atestudiesinmathematics;volume184|Includesbibliographicalreferencesandindex.
Identifiers: LCCN2017016426|ISBN9781470425562(alk. paper)
Subjects: LCSH: Directed graphs. | Representations of graphs. | Vector spaces. | AMS: As-
sociative rings and algebras – Representation theory of rings and algebras – Representations
of Artinian rings. msc | Associative rings and algebras – Representation theory of rings and
algebras–Auslander-Reitensequences(almostsplitsequences)andAuslander-Reitenquivers.
msc | Algebraic geometry – Algebraic groups – Geometric invariant theory. msc | Commu-
tative algebra–Generalcommutativering theory– Actionsofgroupsoncommutativerings;
invarianttheory. msc
Classification: LCCQA166.15.D472017|DDC512/.46–dc23
LCrecordavailableathttps://lccn.loc.gov/2017016426
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Contents
Preface ix
Chapter 1. Introduction 1
§1.1. Basic Definitions and Examples 1
§1.2. The Category of Quiver Representations 3
§1.3. Representation Spaces 6
§1.4. Indecomposable Representations 8
§1.5. The Path Algebra 11
§1.6. Duality 14
§1.7. The Krull-Remak-Schmidt Theorem 15
§1.8. Bibliographical Remarks 17
Chapter 2. Homological Algebra of Quiver Representations 19
§2.1. Projective and Injective Modules 19
§2.2. Projective and Injective Quiver Representations 22
§2.3. The Hereditary Property of Path Algebras 24
§2.4. The Extensions Group 27
§2.5. The Euler Form 32
§2.6. Bibliographical Remarks 34
Chapter 3. Finite Dimensional Algebras 35
§3.1. Quivers with Relations 35
§3.2. The Jacobson Radical 38
§3.3. Basic Algebras 41
v
vi Contents
§3.4. Morita Equivalence 43
§3.5. Bibliographical Remarks 47
Chapter 4. Gabriel’s Theorem 49
§4.1. Quivers of Finite Representation Type 50
§4.2. Dynkin Graphs 52
§4.3. The Reflection Functors 57
§4.4. The Coxeter Functor 64
§4.5. Examples 69
§4.6. Bibliographical Remarks 71
Chapter 5. Almost Split Sequences 73
§5.1. Ideals of Morphisms in the Module Categories 73
§5.2. Irreducible Morphisms 77
§5.3. The Auslander-Reiten Quiver 83
§5.4. The Notion of an Almost Split Sequence 86
§5.5. Bibliographical Remarks 94
Chapter 6. Auslander-Reiten Theory 97
§6.1. Injective Envelopes and Projective Covers 97
§6.2. The Transpose Functor 100
§6.3. The Translation Functor for Quivers 102
§6.4. Auslander-Reiten Duality 103
§6.5. Coxeter Functors Revisited 107
§6.6. The Auslander-Reiten Quiver for Hereditary Algebras 111
§6.7. The Preprojective Algebra 114
§6.8. Bibliographical Remarks 116
Chapter 7. Extended Dynkin Quivers 117
§7.1. Representations of the Kronecker Quiver 118
§7.2. The Auslander-Reiten Quiver of the Kronecker Quiver 121
§7.3. AR Quivers for other Extended Dynkin Types 122
§7.4. Bibliographical Remarks 129
Chapter 8. Kac’s Theorem 131
§8.1. Deformed Preprojective Algebras 131
§8.2. Reflections 136
§8.3. Root Systems 138
Contents vii
§8.4. Quiver Representations over Finite Fields 142
§8.5. Bibliographical Remarks 147
Chapter 9. Geometric Invariant Theory 149
§9.1. Algebraic Group Actions 150
§9.2. Linearly Reductive Groups 155
§9.3. The Geometry of Quotients 162
§9.4. Semi-Invariants and the Sato-Kimura Lemma 164
§9.5. Geometric Invariant Theory 167
§9.6. The Hilbert-Mumford Criterion 169
§9.7. GIT for Quiver Representations 172
§9.8. GIT Quotients with Respect to Weights 176
§9.9. Bibliographical Remarks 182
Chapter 10. Semi-invariants of Quiver Representations 183
§10.1. Background from Classical Invariant Theory 184
§10.2. The Le Bruyn-Procesi Theorem 187
§10.3. Background from the Representation Theory of GL 191
n
§10.4. Semi-invariants and Representation Theory 197
§10.5. Examples for Dynkin Quivers 199
§10.6. Schofield Semi-invariants 204
§10.7. The Main Theorem and Saturation Theorem 206
§10.8. Proof of the Main Theorem 211
§10.9. Semi-invariants for Dynkin Quivers 216
§10.10. Semi-invariants for Extended Dynkin Types 218
§10.11. More Examples of Rings of Semi-invariants 225
§10.12. Schofield Incidence Varieties 231
§10.13. Bibliographical Remarks 240
Chapter 11. Orthogonal Categories and Exceptional Sequences 243
§11.1. Schur Representations 244
§11.2. The Canonical Decomposition 246
§11.3. Tilting Modules 254
§11.4. Orthogonal Categories 259
§11.5. Quivers with Two Vertices 266
§11.6. Two Sincerity Results 269
§11.7. The Braid Group Action on Exceptional Sequences 270
viii Contents
§11.8. Examples 273
§11.9. An Algorithm for the Canonical Decomposition 275
§11.10. Bibliographical Remarks 285
Chapter 12. Cluster Categories 287
§12.1. A Combinatorial Model for Type A 288
n
§12.2. Cluster Combinatorics and Decorated Representations 294
§12.3. Triangulated Categories and Derived Categories 303
§12.4. The Derived Category of Quiver Representations 310
§12.5. Cluster Categories 316
§12.6. Cluster Tilted Algebras 318
§12.7. Bibliographical Remarks 322
Notation 325
Index 327
Bibliography 331
Preface
Theideaofwritingthisbookcamefromthecollaborationoftheauthors. We
startedworkingonquiverrepresentationwhenthefirstauthorvisitedNorth-
eastern University as a Research Scholar in the academic year 1997/1998.
We both came from other fields of algebra so we had to learn the subject
from scratch. At the same time we were interested in connections of quiver
representations with invariant theory, representations of algebraic groups,
and algebraic geometry.
Thisexperiencemadeusrealizethat(exceptforCrawley-Boevey’snotes
[18] available online), there is no introductory text which would allow a
personwithoutanyknowledgeofArtinalgebrastolearnthesubjectquickly.
Later both authors taught courses on quiver representations at Uni-
versity of Michigan and Northeastern University, respectively. The second
author also taught a two month course on quiver representations at Tor
Vergata University in Rome in the spring of 2007.
We are grateful to Calin Chindris, Jiarui Fei, Ryan Kinser, Andrea Ap-
pel, Andrew Carroll, Sachin Gautam, Daniel Labardini, Kavita Sutar, Sal-
vatore Stella, Riccardo Aragona, Ada Boralevi, Cristina DiTrapano, Luca
Moci, and other students who took part in these courses.
The book is addressed to non-specialists, who want to learn the subject
without going through the extended preparation in algebra, just starting
from basic linear algebra. It turned out to be impossible to be completely
elementary, so in some places we use some basic algebraic geometry (mainly
the dimension counts).
ix