Cuadernos de olimpiada combi PabloSoberon

Problem-Solving
Methods in
Combinatorics
Pablo Soberón
An Approach to Olympiad Problems

Problem-Solving Methods in Combinatorics

Pablo Soberón
Problem-Solving
Methods in
Combinatorics
An Approach to Olympiad Problems

Pablo Soberón
Department of Mathematics
University College London
London, UK
ISBN 978-3-0348-0596-4 ISBN 978-3-0348-0597-1 (eBook)
DOI 10.1007/978-3-0348-0597-1
Springer Basel Heidelberg New York Dordrecht London
Library of Congress Control Number: 2013934547
Mathematics Subject Classification: 05-01, 97K20
© Springer Basel 2013
This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of
the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation,
broadcasting, reproduction on microfilms or in any other physical way, and transmission or information
storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology
now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection
with reviews or scholarly analysis or material supplied specifically for the purpose of being entered
and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of
this publication or parts thereof is permitted only under the provisions of the Copyright Law of the
Publisher’s location, in its current version, and permission for use must always be obtained from Springer.
Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations
are liable to prosecution under the respective Copyright Law.
The use of general descriptive names, registered names, trademarks, service marks, 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.
While the advice and information in this book are believed to be true and accurate at the date of pub-
lication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any
errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect
to the material contained herein.
Printed on acid-free paper
Springer Basel is part of Springer Science+Business Media (www.birkhauser-science.com)

Contents
1 First Concepts ............................... 1
1.1 SetsandFirstCountings....................... 1
1.2 Induction ............................... 5
1.3 PathsinBoards ........................... 9
1.4 A Couple of Tricks . . ........................ 12
1.5 Problems............................... 15
2 The Pigeonhole Principle ......................... 17
2.1 The Pigeonhole Principle ...................... 17
2.2 RamseyNumbers .......................... 20
2.3 The Erd˝ os-Szekeres Theorem .................... 22
2.4 An Application in Number Theory . . . .............. 23
2.5 Problems............................... 24
3 Invariants ................................. 27
3.1 Definition and First Examples .................... 27
3.2 Colorings............................... 31
3.3 ProblemsInvolvingGames ..................... 33
3.4 Problems............................... 37
4 Graph Theory ............................... 43
4.1 Basic Concepts ............................ 43
4.2 Connectedness and Trees ...................... 47
4.3 Bipartite Graphs . . . ........................ 51
4.4 Matchings .............................. 53
4.5 Problems............................... 55
5 Functions ................................. 59
5.1 Functions in Combinatorics ..................... 59
5.2 Permutations............................. 63
5.3 CountingTwice ........................... 69
5.4 The Erd˝ os-Ko-Rado Theorem .................... 72
5.5 Problems............................... 74
6 Generating Functions .......................... 77
6.1 Basic Properties . . . ........................ 77
v