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Hybrid Metaheuristics An Emerging Approach to Optimization 123 Dr.ChristianBlum Dr.AndreaRoli ALBCOM DEIS,CampusofCesena Dept.LlenguatgesiSistemesInforma´tics AlmaMaterStudiorum UniversitatPolite`cnicadeCatalunya Universita`diBologna JordiGirona1-3Omega112, ViaVenezia52 CampusNord I-47023Cesena E-08034Barcelona Italy Spain [email protected] [email protected] Dr.MariaJose´BlesaAguilera Dr.MichaelSampels ALBCOM IRIDIA Dept.LlenguatgesiSistemesInforma´tics Universite´LibredeBruxelles UniversitatPolite`cnicadeCatalunya AvenueFranklinRoosevelt50,CP194/6 JordiGirona1-3Omega213, B-1050Brussels CampusNord Belgium E-08034Barcelona [email protected] Spain [email protected] ISBN978-3-540-78294-0 e-ISBN978-3-540-78295-7 StudiesinComputationalIntelligenceISSN1860-949X LibraryofCongressControlNumber:2008921710 (cid:1)c 2008Springer-VerlagBerlinHeidelberg Thisworkissubjecttocopyright.Allrightsarereserved,whetherthewholeorpartofthematerial isconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation,broad- casting,reproductiononmicrofilmorinanyotherway,andstorageindatabanks.Duplicationof thispublicationorpartsthereofispermittedonlyundertheprovisionsoftheGermanCopyrightLaw ofSeptember9,1965,initscurrentversion,andpermissionforusemustalwaysbeobtainedfrom Springer-Verlag.ViolationsareliabletoprosecutionundertheGermanCopyrightLaw. Theuseofgeneraldescriptivenames,registerednames,trademarks,etc.inthispublicationdoesnot imply, even in the absence of a specific statement, that such names are exempt from the relevant protectivelawsandregulationsandthereforefreeforgeneraluse. Coverdesign:Deblik,Berlin,Germany Printedonacid-freepaper 9 8 7 6 5 4 3 2 1 springer.com For my parents Maria and Dieter (Christian Blum) For Christian and Marc (Mar´ıa J. Blesa) For Elisabetta, Raffaella and Paolo (Andrea Roli) For Astrid, Julian and Carlotta (Michael Sampels) Preface Whenfacingcomplexnewoptimizationproblems,itisverynaturaltouserules of thumb, common sense, trial and error, which are called heuristics, in order to find possible answers. Such approaches are, at first sight, quite different from rigorous scientific approaches, which are usually based on characteriza- tions, deductions, hypotheses and experiments. It is common knowledge that many heuristic criteria and strategies, which are used to find good solutions for particular problems, share common aspects and are often independent of the problems themselves. In the computer science and artificial intelligence community, the term metaheuristic was created and is now well accepted for general techniques which are not specific to a particular problem. Genetic and evolutionary al- gorithms, tabu search, simulated annealing, iterated local search, ant colony optimization, scatter search, etc. are typical representatives of this generic term.Researchinmetaheuristicshasbeenveryactiveduringthelastdecades, whichiseasytounderstand,whenlookingatthewidespectrumoffascinating problemsthathavebeensuccessfullytackledandthebeautyofthetechniques, manyoftheminspiredbynature.Eventhoughmanycombinatorialoptimiza- tion problems are very hard to solve optimally, the quality of the results obtained by somewhat unsophisticated metaheuristics is often impressive. This success record has motivated researchers to focus on why a given metaheuristic is successful, on which problem instance characteristics should be exploited and on which problem model is best for the metaheuristic of choice. Investigations on theoretical aspects have begun, and formal theories of the working of metaheuristics are being developed. Questions as to which metaheuristic isbestforagiven problemusedtobequitecommon and,more prosaically, often led to a defensive attitude towards other metaheuristics. Finally, it also became evident that the concentration on a single meta- heuristic is rather restrictive for advancing the state of the art when tack- ling both academic and practical optimization problems. Examples showed that a skillful combination of metaheuristics with concepts originating from other types of algorithms for optimization can lead to more efficient behavior VIII Preface and greater flexibility. For example, the incorporation of typical operations research (OR) techniques, such as mathematical programming, into meta- heuristics may be beneficial. Also, the combination of metaheuristics with other techniques known from artificial intelligence (AI), such as constraint programming and data mining, can be fruitful. Nowadays, such a combina- tion of one metaheuristic with components from other metaheuristics or with techniques from AI and OR techniques is called a hybrid metaheuristic. The lack of a precise definition of the term hybrid metaheuristics is some- times subject to criticism. On the contrary, we believe that this relatively open definition is helpful, because strict borderlines between related fields of research often block creative research directions. A vital research community needs new ideas and creativity, not overly strict definitions and limitations. In 2004, we founded with the First International Workshop on Hybrid Metaheuristics (HM 2004) a series of annual workshops. These workshops have developed into a forum for researchers who direct their work towards hybrid algorithms that go beyond the scope of single metaheuristics. The growing interest in these workshops is an indication that questions regarding the proper integration of different algorithmic components and the adequate analysis of results can now emerge from the shadows. With this backdrop, it becomes evident that hybrid metaheuristics is now a part of experimental science and that its strong interdisciplinarity supports cooperation between researchers with different expertise. In the light of the above, we feel that it is now time for a textbook on hybrid metaheuristics, presenting the most important achievements and de- velopments in this domain. We have invited key experts in the field to supply chapters with the objective of providing an introduction to the themes of hybrid metaheuristics and discussing associated theoretical aspects or appli- cations.Wehopethat,byreadingthisbook,eitherresearchersorstudentswill have an easy entry point to this fascinating field and will get a clear overview of its research directions. Barcelona, Bologna, Brussels Christian Blum November 2007 Maria Jos´e Blesa Aguilera Andrea Roli Michael Sampels Contents Hybrid Metaheuristics: An Introduction Christian Blum and Andrea Roli ................................... 1 Combining (Integer) Linear Programming Techniques and Metaheuristics for Combinatorial Optimization Gu¨nther R. Raidl and Jakob Puchinger ............................. 31 The Relation Between Complete and Incomplete Search Steven Prestwich................................................. 63 Hybridizations of Metaheuristics With Branch & Bound Derivates Christian Blum, Carlos Cotta, Antonio J. Ferna´ndez, Jos´e E. Gallardo, and Monaldo Mastrolilli .......................................... 85 Very Large-Scale Neighborhood Search: Overview and Case Studies on Coloring Problems Marco Chiarandini, Irina Dumitrescu, and Thomas Stu¨tzle ............117 Hybrids of Constructive Metaheuristics and Constraint Programming: A Case Study with ACO Bernd Meyer ....................................................151 Hybrid Metaheuristics for Packing Problems Toshihide Ibaraki, Shinji Imahori, and Mutsunori Yagiura.............185 Hybrid Metaheuristics for Multi-objective Combinatorial Optimization Matthias Ehrgott and Xavier Gandibleux ............................221 Multilevel Refinement for Combinatorial Optimisation: Boosting Metaheuristic Performance Chris Walshaw ..................................................261 Hybrid Metaheuristics: An Introduction Christian Blum1 and Andrea Roli2 1 ALBCOM research group Universitat Polit`ecnica de Catalunya, Barcelona, Spain [email protected] 2 DEIS, Campus of Cesena Alma Mater Studiorum Universit`a di Bologna, Bologna, Italy [email protected] Summary. In many real life settings, high quality solutions to hard optimization problemssuchasflightschedulingorloadbalancingintelecommunicationnetworks arerequiredinashortamountoftime.Duetothepracticalimportanceofoptimiza- tion problems for industry and science, many algorithms to tackle them have been developed. One important class of such algorithms are metaheuristics. The field of metaheuristic research has enjoyed a considerable popularity in the last decades. In this introductory chapter we first provide a general overview on metaheuristics. Thenweturntowardsanewandhighlysuccessfulbranchofmetaheuristicresearch, namely the hybridization of metaheuristics with algorithmic components originat- ingfromothertechniquesforoptimization.Thechapterendswithanoutlineofthe remaining book chapters. 1 Introduction In the context of combinatorial optimization (CO), algorithms can be classi- fied as either complete or approximate algorithms. Complete algorithms are guaranteed to find for every finite size instance of a CO problem an opti- mal solution in bounded time (see [80, 76]). Yet, for CO problems that are NP-hard [42], no polynomial time algorithm exists, assuming that P (cid:1)=NP. Therefore, complete methods might need exponential computation time in the worst-case. This often leads to computation times too high for practi- cal purposes. In approximate methods such as metaheuristics we sacrifice the guarantee of finding optimal solutions for the sake of getting good solutions in a significantly reduced amount of time. Thus, the use of metaheuristics has received more and more attention in the last 30 years. This was also the caseincontinuousoptimization;duetootherreasons:Metaheuristicsareusu- ally easier to implement than classical gradient-based techniques. Moreover, metaheuristics donotrequiregradientinformation. This isconvenient forop- timizationproblemswheretheobjectivefunctionisonlyimplicitlygiven(e.g., C.BlumandA.Roli:HybridMetaheuristics:AnIntroduction,StudiesinComputationalIntel- ligence(SCI)114,1–30(2008) www.springerlink.com (cid:1)c Springer-VerlagBerlinHeidelberg2008 2 Christian Blum and Andrea Roli when objective function values are obtained by simulation), or where the ob- jective function is not differentiable. The first two decades of research on metaheuristics were characterized by the application of rather standard metaheuristics. However, in recent years it has become evident that the concentration on a sole metaheuristic is rather restrictive. A skilled combination of a metaheuristic with other optimization techniques, a so called hybrid metaheuristic, can provide a more efficient be- havior and a higher flexibility when dealing with real-world and large-scale problems. This can be achieved, for instance, by combining the complemen- tary strengths of metaheuristics on one side and of complete methods such as branch & bound techniques or mathematical programming on the other side. In general, hybrid metaheuristic approaches can be classified as either collab- orative combinations or integrative combinations (see [85, 86]). Collaborative combinations are based on the exchange of information between several opti- mizationtechniquesrunsequentially(orinparallel).Thiskindofcombination is more related to cooperative and parallel search and we forward the inter- estedreadertothespecificliteratureonthesubject[55,24,105,98,19,18,3]. Mostcontributionsofthisbookdealwithinterestingandrepresentativecases of integrative combinations. In this introductory chapter we first aim at giving an overview over some of the most important metaheuristics. Then we deal with the hybridization of metaheuristics with other techniques for optimization. Finally, we shortly outline the books’ contents, with the aim of providing a general guidance to the reader. 2 Basics An optimization problem P can be described as a triple (S,Ω,f), where 1. S is the search space defined over a finite set of decision variables X , i i = 1,...,n. In case these variables have discrete domains we deal with discrete optimization (or combinatorial optimization), and in case of con- tinuous domains P is called a continuous optimization problem. Mixed variableproblemsalsoexist.Ω isasetofconstraintsamongthevariables; 2. f :S →IR+ is the objective function that assigns a positive cost value to each element (or solution) of S. The goal is to find a solution s ∈ S such that f(s) ≤ f(s(cid:2)), ∀ s(cid:2) ∈ S (in case wewanttominimizetheobjectivefunction),orf(s)≥f(s(cid:2)),∀s(cid:2) ∈S (incase the objective function must be maximized). In real-life problems the goal is often to optimize several objective functions at the same time. This form of optimization is labelled multi-objective optimization. Approximate methods are generally based on two basic principles: con- structive heuristics and local search methods.
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