Advances in Greedy Algorithms

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Witold Bednorz

The greedy algorithm is one of the simplest approaches to solve the optizmization problem in which we want to determine the global optimum of a given function by a sequence of steps where at each stage we can make a choice among a class of possible decisions. In the greedy method the choice of the optimal decision is made on the information at hand without worrying about the effect these decisions may have in the future. Greedy algorithms are easy to invent, easy to implement and most of the time quite efficient. However there are many problems that cannot be solved correctly by the greedy approach. The common example of the greedy concept is the problem of ‘Making Change’ in which we want to make a change of a given amount using the minimum number of US coins. We can use five different values: dollars (100 cents), quarters (25 cents), dimes (10 cents), nickels (5 cents) and pennies (1 cent). The greedy algorithm is to take the largest possible amount of coins of a given value starting from the highest one (100 cents). It is easy to see that the greedy strategy is optimal in this setting, indeed for proving this it suffices to use the induction principle which works well because in each step either the procedure has ended or there is at least one coin we can use of the actual value. It means that the problem has a certain optimal substructure, which makes the greedy algorithm effective. However a slight modification of ‘Making Change’, e.g. where one value is missing, may turn the greedy strategy to be the worst choice. Therefore there are obvious limits for using the greedy method: whenever there is no optimal substructure of the problem we cannot hope

that the greedy algorithm will work. On the other hand there is a lot of problems where the greedy strategy works unexpectedly well and the purpose of this book is to communicate various results in this area. The key point is the simplicity of the approach which makes the greedy algorithm a natural first choice to analyze the given problem. In this book there are discussed several algorithmic questions in: biology, combinatorics, networking, scheduling

or even pure mathematics, where the greedy algorithm can be used to produce the optimal or nearly optimal answer.

The book was written in 2008 by the numerous authors who contributed the publication by presenting their researches in a form of a self-contained chapters. The idea was to coordinate the international project where specialists all over the world can share their knowledge on the greedy algorithms theory. Each chapter comprises a separate study on some optimization problem giving both an introductory look into the theory the problem comes from and some new developments invented by author(s). Usually some elementary knowledge is assumed, yet all the required facts are quoted mostly in examples, remarks or theorems. The publication may be useful for all graduates and undergraduates interested in

the algorithmic theory with the focus on the greedy approach and applications of this method to various concrete examples. Most of scientists involved in the project are young at the full strength of their career, hence the presented content is fresh and acquaints with the new directions where the theory of greedy algorithms evolves to.

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