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Cover Page
Texts in Theoretical Computer Science
Title: Extremal Combinatorics With Applications in Computer Science, Second Edition
ISBN 9783642173639
Preface
Preface to the First Edition
Preface to the Second Edition
Notation
Contents
Notation
Part I The Classics
1 Counting.
2 AdvancedCounting.
3 Probabilistic Counting
4 The Pigeonhole Principle
5 Systems of Distinct Representatives
Part II Extremal Set Theory
6 Sunflowers
7 Intersecting Families
8 Chains and Antichains
9 Blocking Sets and the Duality
10 Density and Universality
11 Witness Sets and Isolation
12 Designs
Part III The Linear Algebra Method
13 The Basic Method
14 Orthogonality and Rank Arguments
15 Eigenvalues and Graph Expansion
16 The Polynomial Method
17 Combinatorics of Codes
Part IV The Probabilistic Method
18 Linearity of Expectation
19 The Lovász Sieve
20 The Deletion Method
21 The Second Moment Method
22 The Entropy Function
23 Random Walks
24 Derandomization
Part V Fragments of Ramsey Theory
25 Ramseyan Theorems for Numbers
26 The Hales–Jewett Theorem
27 Applications in Communication Complexity
References
Index
Part I The Classics
1. Counting
1.1 The binomial theorem
1.2 Selection with repetitions
1.3 Partitions
1.4 Double counting
1.5 The averaging principle
1.6 The inclusion-exclusion principle
Exercises
2. Advanced Counting
2.1 Bounds on intersection size
2.2 Graphs with no 4-cycles
2.3 Graphs with no induced 4-cycles
2.4 Zarankiewicz’s problem
2.5 Density of 0-1 matrices
2.6 The Lovász–Stein theorem
Exercises
3. Probabilistic Counting
3.1 Probabilistic preliminaries
3.2 Tournaments
3.3 Universal sets
3.4 Covering by bipartite cliques
3.5 2-colorable families
3.6 The choice number of graphs
Exercises
4. The Pigeonhole Principle
4.1 Some quickies
4.2 The Erdős–Szekeres theorem
4.3 Mantel’s theorem
4.4 Turán’s theorem
4.5 Dirichlet’s theorem
4.6 Swell-colored graphs
4.7 The weight shifting argument
4.8 Schur’s theorem
4.9 Ramseyan theorems for graphs
4.10 Ramsey’s theorem for sets
Exercises
5. Systems of Distinct Representatives
5.1 The marriage theorem
5.2 Two applications
5.3 Min–max theorems
5.4 Matchings in bipartite graphs
Exercises
Part II Extremal Set Theory
6. Sunflowers
6.1 The sunflower lemma
6.2 Modifications
6.3 Applications
Exercises
7. Intersecting Families
7.1 Ultrafilters and Helly property
7.2 The Erdős–Ko–Rado theorem
7.3 Fisher’s inequality
7.4 Maximal intersecting families
7.5 Cross-intersecting families
Exercises
8. Chains and Antichains
8.1 Decomposition in chains and antichains
8.2 Application: the memory allocation problem
8.3 Sperner’s theorem
8.4 The Bollobás theorem
8.5 Strong systems of distinct representatives
8.6 Union-free families
Exercises
9. Blocking Sets and the Duality
9.1 Duality
9.2 The blocking number
9.3 Helly-type theorems
9.4 Blocking sets and decision trees
9.5 Blocking sets and monotone circuits
Exercises
10. Density and Universality
10.1 Dense sets
10.2 Hereditary sets
10.3 Matroids and approximation
10.4 The Kruskal–Katona theorem
10.5 Universal sets
10.6 Paley graphs
10.7 Full graphs
Exercises
11. Witness Sets and Isolation
11.1 Bondy’s theorem
11.2 Average witnesses
11.3 The isolation lemma
11.4 Isolation in politics: the dictator paradox
Exercises
12. Designs
12.1 Regularity
12.2 Finite linear spaces
12.3 Difference sets
12.4 Projective planes
12.5 Resolvable designs
Exercises
Part III The Linear Algebra Method
13. The Basic Method
13.1 The linear algebra background
13.2 Graph decompositions
13.3 Inclusion matrices
13.4 Disjointness matrices
13.5 Two-distance sets
13.6 Sets with few intersection sizes
13.7 Constructive Ramsey graphs
13.8 Zero-patterns of polynomials
Exercises
14. Orthogonality and Rank Arguments
14.1 Orthogonal coding
14.2 Balanced pairs
14.3 Hadamard matrices
14.4 Matrix rank and Ramsey graphs
14.5 Lower bounds for boolean formulas
Exercises
15. Eigenvalues and Graph Expansion
15.1 Expander graphs
15.2 Spectral gap and the expansion
15.3 Expanders and derandomization
Exercises
16. The Polynomial Method
16.1 DeMillo–Lipton–Schwartz–Zippel lemma
16.2 Solution of Kakeya’s problem in finite fields
16.3 Combinatorial Nullstellensatz
Exercises
17. Combinatorics of Codes
17.1 Error-correcting codes
17.2 Bounds on code size
17.3 Linear codes
17.4 Universal sets from linear codes
17.5 Spanning diameter
17.6 Expander codes
17.7 Expansion of random graphs
Exercises
Part IV The Probabilistic Method
18. Linearity of Expectation
18.1 Hamilton paths in tournaments
18.2 Sum-free sets
18.3 Dominating sets
18.4 The independence number
18.5 Crossings and incidences
18.6 Far away strings
18.7 Low degree polynomials
18.8 Maximum satisfiability
18.9 Hash functions
18.10 Discrepancy
18.11 Large deviation inequalities
Exercises
19. The Lovász Sieve
19.1 The Lovász Local Lemma
19.2 Disjoint cycles
19.3 Colorings
19.4 The k-SAT problem
Exercises
20. The Deletion Method
20.1 Edge clique covering
20.2 Independent sets
20.3 Coloring large-girth graphs
20.4 Point sets without obtuse triangles
20.5 Affine cubes of integers
Exercises
21. The Second Moment Method
21.1 The method
21.2 Distinct sums
21.3 Prime factors
21.4 Separators
21.5 Threshold for cliques
Exercises
22. The Entropy Function
22.1 Quantifying information
22.2 Limits to data compression
22.3 Shannon entropy
22.4 Subadditivity
22.5 Combinatorial applications
Exercises
23. Random Walks
23.1 The satisfiability problem
23.2 Random walks in linear spaces
23.3 Random walks and derandomization
Exercises
24. Derandomization
24.1 The method of conditional probabilities
24.2 The method of small sample spaces
24.3 Sum-free sets: the algorithmic aspect
Exercises
Part V Fragments of Ramsey Theory
25. Ramseyan Theorems for Numbers
25.1 Arithmetic progressions
25.2 Szemerédi’s cube lemma
25.3 Sum-free sets
25.4 Sum-product sets
Exercises
26. The Hales–Jewett Theorem
26.1 The theorem and its consequences
26.2 Shelah’s proof of HJT
Exercises
27. Applications in Communication Complexity
27.1 Multi-party communication
27.2 The hyperplane problem
27.3 The partition problem
27.4 Lower bounds via discrepancy
27.5 Making non-disjoint coverings disjoint
Exercises
References
Index
Symbols
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
Z
Texts in Theoretical Computer Science An EATCS Series Editors: J. Hromkoviˇc G. Rozenberg A. Salomaa Founding Editors: W. Brauer G. Rozenberg A. Salomaa On behalf of the European Association for Theoretical Computer Science (EATCS) Advisory Board: G. Ausiello M. Broy C.S. Calude A. Condon D. Harel M. Nivat C. Papadimitriou D. Scott J. Hartmanis T. Henzinger T. Leighton For further volumes: www.springer.com/series/3214
Stasys Jukna Extremal Combinatorics With Applications in Computer Science Second Edition
Prof. Dr. Stasys Jukna Goethe Universität Frankfurt Institut für Informatik Robert-Mayer Str. 11-15 60054 Frankfurt am Main Germany jukna@thi.informatik.uni-frankfurt.de Vilnius University Institute of Mathematics and Informatics Akademijos 4 08663 Vilnius Lithuania Series Editors Prof. Dr. Juraj Hromkoviˇc ETH Zentrum Department of Computer Science Swiss Federal Institute of Technology 8092 Zürich, Switzerland juraj.hromkovic@inf.ethz.ch Prof. Dr. Arto Salomaa Turku Centre of Computer Science Lemminkäisenkatu 14 A 20520 Turku, Finland asalomaa@utu.fi Prof. Dr. Grzegorz Rozenberg Leiden Institute of Advanced Computer Science University of Leiden Niels Bohrweg 1 2333 CA Leiden, The Netherlands rozenber@liacs.nl ISSN 1862-4499 ISBN 978-3-642-17363-9 DOI 10.1007/978-3-642-17364-6 Springer Heidelberg Dordrecht London New York Texts in Theoretical Computer Science. An EATCS Series e-ISBN 978-3-642-17364-6 Library of Congress Control Number: 2011937551 ACM Codes: G.2, G.3, F.1, F.2, F.4.1 © Springer-Verlag Berlin Heidelberg 2001, 2011 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: KünkelLopka GmbH, Heidelberg Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)
To Indr˙e
Preface Preface to the First Edition Combinatorial mathematics has been pursued since time immemorial, and at a reasonable scientific level at least since Leonhard Euler (1707–1783). It rendered many services to both pure and applied mathematics. Then along came the prince of computer science with its many mathematical problems and needs – and it was combinatorics that best fitted the glass slipper held out. Moreover, it has been gradually more and more realized that combinatorics has all sorts of deep connections with “mainstream areas” of mathematics, such as algebra, geometry and probability. This is why combinatorics is now a part of the standard mathematics and computer science curriculum. This book is as an introduction to extremal combinatorics – a field of com- binatorial mathematics which has undergone a period of spectacular growth in recent decades. The word “extremal” comes from the nature of problems this field deals with: if a collection of finite objects (numbers, graphs, vectors, sets, etc.) satisfies certain restrictions, how large or how small can it be? For example, how many people can we invite to a party where among each three people there are two who know each other and two who don’t know each other? An easy Ramsey-type argument shows that at most five persons can attend such a party. Or, suppose we are given a finite set of nonzero integers, and are asked to mark an as large as possible subset of them under the restriction that the sum of any two marked integers cannot be marked. It turns out that (independent of what the given integers actually are!) we can always mark at least one-third of them. Besides classical tools, like the pigeonhole principle, the inclusion-exclusion principle, the double counting argument, induction, Ramsey argument, etc., some recent weapons – the probabilistic method and the linear algebra method – have shown their surprising power in solving such problems. With a mere knowledge of the concepts of linear independence and discrete prob- ability, completely unexpected connections can be made between algebra,
viii Preface probability, and combinatorics. These techniques have also found striking ap- plications in other areas of discrete mathematics and, in particular, in the theory of computing. Nowadays we have comprehensive monographs covering different parts of extremal combinatorics. These books provide an invaluable source for stu- dents and researchers in combinatorics. Still, I feel that, despite its great po- tential and surprising applications, this fascinating field is not so well known for students and researchers in computer science. One reason could be that, being comprehensive and in-depth, these monographs are somewhat too dif- ficult to start with for the beginner. I have therefore tried to write a “guide tour” to this field – an introductory text which should - be self-contained, - be more or less up-to-date, - present a wide spectrum of basic ideas of extremal combinatorics, - - be accessible to graduate and motivated undergraduate students in show how these ideas work in the theory of computing, and mathematics and computer science. Even if not all of these goals were achieved, I hope that the book will at least give a first impression about the power of extremal combinatorics, the type of problems this field deals with, and what its methods could be good for. This should help students in computer science to become more familiar with combinatorial reasoning and so be encouraged to open one of these monographs for more advanced study. Intended for use as an introductory course, the text is, therefore, far from being all-inclusive. Emphasis has been given to theorems with elegant and beautiful proofs: those which may be called the gems of the theory and may be relatively easy to grasp by non-specialists. Some of the selected arguments are possible candidates for The Book, in which, according to Paul Erdős, God collects the perfect mathematical proofs.∗ I hope that the reader will enjoy them despite the imperfections of the presentation. A possible feature and main departure from traditional books in combina- torics is the choice of topics and results, influenced by the author’s twenty years of research experience in the theory of computing. Another departure is the inclusion of combinatorial results that originally appeared in computer science literature. To some extent, this feature may also be interesting for students and researchers in combinatorics. In particular, some impressive applications of combinatorial methods in the theory of computing are dis- cussed. Teaching. The text is self-contained. It assumes a certain mathematical maturity but no special knowledge in combinatorics, linear algebra, prob- ∗ “You don’t have to believe in God but, as a mathematician, you should believe in The Book.” (Paul Erdős) For the first approximation see M. Aigner and G.M. Ziegler, Proofs from THE BOOK. Second Edition, Springer, 2000.
Preface ix ability theory, or in the theory of computing — a standard mathematical background at undergraduate level should be enough to enjoy the proofs. All necessary concepts are introduced and, with very few exceptions, all results are proved before they are used, even if they are indeed “well-known.” For- tunately, the problems and results of combinatorics are usually quite easy to state and explain, even for the layman. Its accessibility is one of its many appealing aspects. The book contains much more material than is necessary for getting ac- quainted with the field. I have split it into relatively short chapters, each devoted to a particular proof technique. I have tried to make the chapters almost independent, so that the reader can choose his/her own order to fol- low the book. The (linear) order, in which the chapters appear, is just an extension of a (partial) order, “core facts first, applications and recent devel- opments later.” Combinatorics is broad rather than deep, it appears in dif- ferent (often unrelated) corners of mathematics and computer science, and it is about techniques rather than results – this is where the independence of chapters comes from. Each chapter starts with results demonstrating the particular technique in the simplest (or most illustrative) way. The relative importance of the topics discussed in separate chapters is not reflected in their length – only the topics which appear for the first time in the book are dealt with in greater detail. To facilitate the understanding of the material, over 300 exercises of varying difficulty, together with hints to their solution, are included. This is a vital part of the book – many of the examples were chosen to complement the main narrative of the text. Some of the hints are quite detailed so that they actually sketch the entire solution; in these cases the reader should try to fill out all missing details. Acknowledgments. I would like to thank everybody who was directly or indirectly involved in the process of writing this book. First of all, I am grateful to Alessandra Capretti, Anna Gál, Thomas Hofmeister, Daniel Kral, G. Murali Krishnan, Martin Mundhenk, Gurumurthi V. Ramanan, Martin Sauerhoff and P.R. Subramania for comments and corrections. Although not always directly reflected in the text, numerous earlier discus- sions with Anna Gál, Pavel Pudlák, and Sasha Razborov on various combina- torial problems in computational complexity, as well as short communications with Noga Alon, Aart Blokhuis, Armin Haken, Johan Håstad, Zoltan Füredi, Hanno Lefmann, Ran Raz, Mike Sipser, Mario Szegedy, and Avi Wigder- son, have broadened my understanding of things. I especially benefited from the comments of Aleksandar Pekec and Jaikumar Radhakrishnan after they tested parts of the draft version in their courses in the BRICS International Ph.D. school (University of Aarhus, Denmark) and Tata Institute (Bombay, India), and from valuable comments of László Babai on the part devoted to the linear algebra method. I would like to thank the Alexander von Humboldt Foundation and the German Research Foundation (Deutsche Forschungsgemeinschaft) for sup-
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