The editors and authors dedicate this book to Bernhard Korte on the occasion of his seventieth birthday. We, the editors, are happy about the overwhelming feedback to our initiative to honor him with this book and with a workshop in Bonn on November 3-7,2008.Althoughthiswouldbeareasontolookback,wewouldratherliketolook forward and see what are the interesting research directions today. This book is written by leading experts in combinatorial optimization. All - pers were carefully reviewed, and eventually twenty-three of the invited papers were accepted for this book. The breadth of topics is typical for the eld: combinatorial optimization builds bridges between areas like combinatorics and graph theory, submodular functions and matroids, network ows and connectivity, approximation algorithms and mat- matical programming, computational geometry and polyhedral combinatorics. All these topics are related, and they are all addressed in this book. Combi- torial optimization is also known for its numerous applications. To limit the scope, however, this book is not primarily about applications, although some are mentioned at various places. Most papers in this volume are surveys that provide an excellent overview of an activeresearcharea,butthisbookalsocontainsmanynewresults.Highlightingmany of the currently most interesting research directions in combinatorial optimization, we hope that this book constitutes a good basis for future research in these areas.

This volume honours the eminent mathematicians Vera Sos and Andras Hajnal. The book includes survey articles reviewing classical theorems, as well as new, state-of-the-art results. Also presented are cutting edge expository research papers with new theorems and proofs in the area of the classical Hungarian subjects, like extremal combinatorics, colorings, combinatorial number theory, etc. The open problems and the latest results in the papers are sure to inspire further research.

Paul Erdos was one of the most influential mathematicians of the twentieth century, whose work in number theory, combinatorics, set theory, analysis, and other branches of mathematics has determined the development of large areas of these fields. In 1999, a conference was organized to survey his work, his contributions to mathematics, and the far-reaching impact of his work on many branches of mathematics. On the 100th anniversary of his birth, this volume undertakes the almost impossible task to describe the ways in which problems raised by him and topics initiated by him (indeed, whole branches of mathematics) continue to flourish. Written by outstanding researchers in these areas, these papers include extensive surveys of classical results as well as of new developments."

This volume honours the eminent mathematicians Vera Sos and Andras Hajnal. The book includes survey articles reviewing classical theorems, as well as new, state-of-the-art results. Also presented are cutting edge expository research papers with new theorems and proofs in the area of the classical Hungarian subjects, like extremal combinatorics, colorings, combinatorial number theory, etc. The open problems and the latest results in the papers are sure to inspire further research.

Oh cieca cupidigia, oh ira folie, Che si ci sproni nella vita corta, E nell' eterna poi si mal c'immolle! o blind greediness and foolish rage, That in our fleeting life so goads us on And plunges us in boiling blood for ever! Dante, The Divine Comedy Inferno, XII, 17, 49/51. On an afternoon hike during the second Oberwolfach conference on Mathematical Programming in January 1981, two of the authors of this book discussed a paper by another two of the authors (Korte and Schrader [1981]) on approximation schemes for optimization problems over independence systems and matroids. They had noticed that in many proofs the hereditary property of independence systems and matroids is not needed: it is not required that every subset of a feasible set is again feasible. A much weaker property is sufficient, namely that every feasible set of cardinality k contains (at least) one feasible subset of cardinality k - 1. We called this property accessibility, and that was the starting point of our investigations on greedoids.

Since the publication of the first edition of our book, geometric algorithms and combinatorial optimization have kept growing at the same fast pace as before. Nevertheless, we do not feel that the ongoing research has made this book outdated. Rather, it seems that many of the new results build on the models, algorithms, and theorems presented here. For instance, the celebrated Dyer-Frieze-Kannan algorithm for approximating the volume of a convex body is based on the oracle model of convex bodies and uses the ellipsoid method as a preprocessing technique. The polynomial time equivalence of optimization, separation, and membership has become a commonly employed tool in the study of the complexity of combinatorial optimization problems and in the newly developing field of computational convexity. Implementations of the basis reduction algorithm can be found in various computer algebra software systems. On the other hand, several of the open problems discussed in the first edition are still unsolved. For example, there are still no combinatorial polynomial time algorithms known for minimizing a submodular function or finding a maximum clique in a perfect graph. Moreover, despite the success of the interior point methods for the solution of explicitly given linear programs there is still no method known that solves implicitly given linear programs, such as those described in this book, and that is both practically and theoretically efficient. In particular, it is not known how to adapt interior point methods to such linear programs.

Hungarian mathematics has always been known for discrete mathematics, including combinatorial number theory, set theory and recently random structures, and combinatorial geometry. The recent volume contains high level surveys on these topics with authors mostly being invited speakers for the conference "Horizons of Combinatorics" held in Balatonalmadi, Hungary in 2006. The collection gives an overview of recent trends and results in a large part of combinatorics and related topics.

The editors and authors dedicate this book to Bernhard Korte on the occasion of his seventieth birthday. We, the editors, are happy about the overwhelming feedback to our initiative to honor him with this book and with a workshop in Bonn on November 3-7,2008.Althoughthiswouldbeareasontolookback, wewouldratherliketolook forward and see what are the interesting research directions today. This book is written by leading experts in combinatorial optimization. All - pers were carefully reviewed, and eventually twenty-three of the invited papers were accepted for this book. The breadth of topics is typical for the eld: combinatorial optimization builds bridges between areas like combinatorics and graph theory, submodular functions and matroids, network ows and connectivity, approximation algorithms and mat- matical programming, computational geometry and polyhedral combinatorics. All these topics are related, and they are all addressed in this book. Combi- torial optimization is also known for its numerous applications. To limit the scope, however, this book is not primarily about applications, although some are mentioned at various places. Most papers in this volume are surveys that provide an excellent overview of an activeresearcharea, butthisbookalsocontainsmanynewresults.Highlightingmany of the currently most interesting research directions in combinatorial optimization, we hope that this book constitutes a good basis for future research in these area

The aim of this book is NOT to cover discrete mathematics in depth. Rather, it discusses a number of selected results and methods, mostly from the areas of combinatorics and graph theory, along with some elementary number theory and combinatorial geometry. The authors develop most topics to the extent that they can describe the discrete mathematics behind an important application of mathematics such as discrete optimization problems, the Law of Large Numbers, cryptography, and coding to name a few. Another feature that is not covered in other discrete mathematics books is the use of ESTIMATES (How many digits does 100! have? or Which is larger: 2100 or 100!?). There are questions posed in the text and problems at the end of each chapter with solutions for many of them at the end of the book. The book is based on a course taught for several years by two of the authors at Yale University.

Die diskrete Mathematik ist im Begriff, zu einem der wichtigsten Gebiete der mathematischen Forschung zu werden mit Anwendungen in der Kryptographie, der linearen Programmierung, der Kodierungstheorie und Informatik. Dieses Buch richtet sich an Studenten der Mathematik und Informatik, die ein Gefuhl dafur entwickeln mochten, worumes in derMathematik geht, wobei Mathematik hilfreich sein kann, und mit welcher Art Fragen sich Mathematiker auseinandersetzen.

Die Autoren stellen eine Anzahl ausgewahlter Ergebnisse und Methoden der diskreten Mathematik vor, hauptsachlich aus den Bereichen Kombinatorik und Graphentheorie, teilweise aber auch aus der Zahlentheorie, der Wahrscheinlichkeitsrechnung und der kombinatorischen Geometrie.Wo immer es moglich war, haben die Autoren Beweise und Problemlosungen verwendet, um den Studenten zu helfen, die Losungen der Fragestellungen zu verstehen.

Zusatzlich ist eine Vielzahl von Beispielen, Bildern und Ubungsaufgaben uber das Buch verteilt.

Laszlo Lovasz isteiner der Leiter der theoretischen Forschungsabteilung der Microsoft Corporation. Er hat 1999 den Wolf-Preis sowie den Godel-Preis fur die beste wissenschaftliche Veroffentlichung in der Informatik erhalten. Jozsef Pelikan ist Professor am Institut fur Algebra und Zahlentheorie der Eotvos Lorand Universitat in Budapest. Katalin Vesztergombi ist "Senior Lecturer" am Fachbereich Mathematik der Universitat von Washington in Seattle."