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Our Academic and Professional Team
| Professor Said Salhi | Professor of Management Science and Operational Research |
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Profile sectionsTeaching Group: Management Science & Operations Room: Room G01, R&D building Extension: 4672 Email: S.Salhi@kent.ac.uk Office hours: |
| Biography | |
Said Salhi is Professor in Operational Research/Management Science at the Kent Business School (UK) and Director of the Centre for Logistics & Heuristic Optimisation (CLHO) which he established. Prior to his appointment to Kent in 2005, Said was at the University of Birmingham in the School of Mathematics for 15 years. He obtained his BSc in Mathematics at Algiers's University, MSc and PhD in OR in Southampton and Lancaster respectively. Said has edited 3 special issues, chaired the European Working Group in Location Analysis in 1996, and published over 70 papers in academic journals (excluding edited proceedings and book chapters). Said's H-index is 17 from Web of Knowledge (26 from Google Scholar). He has a rich experience in PhD supervision with 21 PhD students successfully passed coming from various countries, culture, religion, colour and gender. Said is a fellow of the OR Society, a member of COPIOR, the IMA, INFORMS and CILT and associate editor of the IMA Journal of Management Mathematics. |
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| Research interests | |
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| Research supervisees | |
Sola Baale Abdalla Elshaikh Chandra Ade Irawan |
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| Past Research supervisees | |
Nurul Huda Mohamed (2012) Abdullah Alharbi (2010) Shaikh Arifusalam (2010) Mutaz Hajarat (2010) Martino Luis (2008) Arif Imran (2008) A. Shuib (2007) Claudia Reyles-Montrel (2005) Robert Smithie (09/2000-09/2004) Abdelrahman Al-Kedhairi (05/2000-08/2004) Zaitul Marlizawati (09/2000-07/2004) The Capacitated Continuous Location-Allocation Problem using constructive and tabu search; [Malaysian Government]. Samuel K Amponsah (01/2000-09/2003) Robert H Currie (12/1999-12/2002) M. M. Mwembeshi (1999-2003) T.Lee (1999) Anne C Wade (10/1998-05/2002) Danil Gamal (10/1997-05/2001) Russel Petch (10/1997-03/2001) Steve Welch (10/1994-07/1999) Charlie James (10/1995-05/1999) Samresh Khan (1997) Mark Agar (10/1994-11/1997) Gabor Nagy (10/1993-08/1996) Paul Thomas (10/1993-07/1996) Craig Robertson (1992) |
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| Publications | |
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Also view these in the Kent Academic Repository
Edited Books
Salhi, S. (2006)
2006 Bath Conference Keynote Papers (OR 48).
Operational Research Society
Salhi, S. and Drezner, Z. (2006)
Location Analysis: theory and Applications.
IMA Journal of Management Mathematics, 17. Oxford Hournals
Book Sections
Salhi, S. and Farber, G. and Coves, A.M. (2008)
Semi Dynamic Demand in a Non-Permutation Flowshop with Constrained Resequencing Buffers.
In: Lirkov, I. and Margenov, S. and Wasniewski, J. Large-scale Scientific Computing. Lecture Notes in Computer Science. Lecture Notes in Computer Science, 4818 (8). Springer-Verlag, Berlin, pp. 536-544. ISBN 9783540788256.
Abstract This work presents the performance comparison of two conceptually different approaches for a mixed model non-permutation flowshop production line. The demand is a semi-dynamic demand with a fixed job sequence for the first station. Resequencing is permitted where stations have access to intermediate or centralized resequencing buffers. The access to the buffers is restricted by the number of available buffer places and the physical size of the products. An exact approach, using Constraint Logic Programming (CLP), and a heuristic approach, a Genetic Algorithm (GA), were applied.
Salhi, S. (2006)
Heuristic Search: The Science of Tomorrow.
In: Salhi, S. OR48 Keynotes Papers. Operational Research Society, pp. 39 - 58.
Salhi, S. and Smithies, R. and Queen, N.M. (2005)
Predicting Colorectal Cancer Recurrence: A Metaheuristic Approach.
In: Tibaraki, K.N. Metaheuristic: Progress as Real Problem Solvers. Kluwer, pp. 259-286. ISBN 978-0387253824.
Wade, A. and Salhi, S. (2003)
An ant system algorithm for the mixed vehicle routing problem with backhauls.
In: Resende, M.G.C. and Sousa, J.P.de Metaheuristics: Computer Decision-Making. Applied Optimization. Kluwer academic Publishers, AH Dordrecht, The Netherlands, pp. 699-719. ISBN 978-1402076534.
Articles
Plastino, A. and Fuchshuber, R. and Martins, S.de.L et al. (2011)
A hybrid data mining metaheuristic for the p-median problem.
Statistical Analysis & Data Mining Journal, 4 (3). pp. 313-355.
Abstract Metaheuristics represent an important class of techniques to solve, approximately, hard combinatorial optimization problems for which the use of exact methods is impractical. In this work, we propose a hybrid version of the Greedy Randomized Adaptive Search Procedures (GRASP) metaheuristic, which incorporates a data mining process, to solve the p-median problem. We believe that patterns obtained by a data mining technique, from a set of suboptimal solutions of a combinatorial optimization problem, can be used to guide metaheuristic procedures in the search for better solutions. Traditional GRASP is an iterative metaheuristic which returns the best solution reached over all iterations. In the hybrid GRASP proposal, after executing a significant number of iterations, the data mining process extracts patterns from an elite set of suboptimal solutions for the p-median problem. These patterns present characteristics of near optimal solutions and can be used to guide the following GRASP iterations in the search through the combinatorial solution space. Computational experiments, comparing traditional GRASP and different data mining hybrid proposals for the p-median problem, showed that employing patterns mined from an elite set of suboptimal solutions made the hybrid GRASP find better results. Besides, the conducted experiments also evidenced that incorporating a data mining technique into a metaheuristic accelerated the process of finding near optimal and optimal solutions. © 2011 Wiley Periodicals, Inc. Statistical Analysis and Data Mining 2011
Moin, N.H. and Salhi, S. and Aziz, N.A.B (2011)
An efficient hybrid genetic algorithm for the multi-product multi-period inventory routing problem.
International Journal of Production Economics (IJPE), 133 (1). pp. 334-343. ISSN 0925-5273.
Abstract The inventory routing problem (IRP) addressed in this study is a many-to-one distribution network consisting of an assembly plant and many distinct suppliers where each supplies a distinct product. We consider a finite horizon, multi-periods, multi-suppliers and multi-products where a fleet of capacitated homogeneous vehicles, housed at a depot, transport products from the suppliers to meet the demand specified by the assembly plant in each period. The demand for each product is deterministic and time varying. A mathematical formulation of the problem is given and CPLEX 9.1 is run for a finite amount of time to obtain lower and upper bounds. A hybrid genetic algorithm, which is based on the allocation first route second strategy and which considers both the inventory and the transportation costs, is proposed. In addition to a new set of crossover and mutation operators, we also introduce two new chromosome representations. Several medium and small sized problems are also constructed and added to the existing data sets to show the effectiveness of the proposed approach.
Garcia-Villoria, Alberto and Salhi, S. and Corominas, A. et al. (2011)
Hyper-heuristic approaches for the response time variability problem.
European Journal of Operational Research, 211 (1). pp. 160-169. ISSN 0377-2217.
Abstract We propose two classes for the implementation of hyper-heuristic algorithms. The first is based on constructive heuristics, whereas the second uses improvement methods. Within the latter class, a general framework is designed for the use of local search procedures and metaheuristics as low-level heuristics. A dynamic scheme to guide the use of these approaches is also devised. These ideas are tested on an NP-hard scheduling problem known as the response time variability problem (RTVP). An intensive computational experiment shows, especially in the second class where the new best results are found, the effectiveness of the proposed hyper-heuristics.
Martino, L. and Salhi, S. and Nagy, G. (2011)
A guided reactive GRASP for the capacitated multi-source Weber problem.
Computers & Operations Research, 38 (7). pp. 1014-1024. ISSN 0305-0548.
Abstract The capacitated multi-source Weber problem entails finding both the locations of capacitated facilities on a plane and their customer allocations. A framework that uses adaptive learning and functional representation to construct the restricted candidate list (RCL) within a greedy randomized adaptive search procedure (GRASP) is put forward. An implementation of restricted regions that forbids new facilities to be located too close to the previously found facilities is also embedded into the search to build up the RCL more efficiently. The performance of this GRASP based approach is tested on three classes of instances with constant and variable capacities. Very competitive results are obtained when compared to the best known results from the literature.
Farber, G. and Salhi, S. and Coves, A.M. (2010)
Performance evaluation of hybrid-CLP vs GA: Non permutation flowshop with constrained resequencing buffers.
International Journal of Manufacturing & Management, 20 (1/4). pp. 242-258. ISSN 1368-2148.
Abstract This paper is located in the area of mixed model non-permutation flowshop production lines where jobs of more than one model are being processed on the same production line in an arbitrary sequence. Nevertheless, the majority of publications in this area are limited to solutions which determine the job sequence before the jobs enters the line and maintains it without interchanging jobs until the end of the production line, which is known as permutation flowshop. The present work considers a non-permutation flowshop. Resequencing is permitted where stations have access to intermediate or centralised resequencing buffers. The access to the buffers is restricted by the number of available buffer places and the physical size of the products. The primary objective is the minimisation of the make span, but also setup-cost and setup-time is contemplated. A hybrid approach, using constraint logic programming (CLP), is presented and compared to a genetic algorithm (GA). These solution methods are conceptually different and recommendations for their applicability are presented.
Salhi, S. and Alkhedairi, A. (2010)
Integrating heuristic information into exact methods: the case of the vertex p-centre problem.
Journal of the Operational Research Society, 61 (11). pp. 1619-1631. ISSN 0160-5682.
Abstract We solve the vertex p-centre problem optimally using an exact method that considers both upper and lower bounds as part of its search engine. Tight upper bounds are generated quickly via an efficient three-level heuristic, which are then used to derive potential lower bounds accordingly. These two pieces of information when used together make our chosen exact method more efficient at obtaining optimal solutions relatively quickly. The proposed implementation produced excellent results when tested on the OR Library data set. This integrated approach can be adopted for those exact methods that consider both upper and lower bounds within their search engine and hence provide a wider spectrum of applicability in other hard combinatorial problems.
Luis, M. and Salhi, S. and Nagy, G. (2009)
Region-Rejection Based Heuristics for the Capacitated Multi-Source Weber Problem.
Computers and Operations Research, 36 (6). pp. 2007-2017. ISSN 0305-0548.
Abstract A new type of constructive and adaptive heuristics is put forward to generate initial solutions for the capacitated multi-source Weber problem. This technique is based on guiding the search by constructing restricted regions that forbid new locations to be sited too close to the previously found locations In. this work, a restricted region is represented by a circle whose radius is initially set to a fixed value, based on the sparsity of the customers and the number of facilities, and then a scheme that dynamically adjusts the radius at each facility is proposed. A discretisation technique that divides a continuous space into a discrete number of cells while embedding the use of restricted regions within the search is also presented. The experiments show that the proposed region-rejection methods, though simple and easy to understand, provide encouraging results with regard to both solution quality and computational effort. Some future research avenues are also briefly highlighted. (C) 2008 Elsevier Ltd. All rights reserved.
Salhi, S. and Nagy, G. (2009)
Local improvement in plananr facility location using vehicle routing.
Ann OR, 167 (1). pp. 287-296. ISSN 0254-5330.
Abstract In physical distribution the location of depots and vehicle routes are interdependent problems, but they are usually treated independently. Location-routing is the study of solving locational problems such that routing considerations are taken into account. We present an iterative heuristic for the location-routing problem on the plane. For each depot the Weber problem is solved using the end-points of the routes found previously as input nodes to the Weiszfeld procedure. Although the improvements found are usually small they show that it pays not to ignore the routing aspects when solving continuous location problems. Possible research avenues in continuous location-routing will also be suggested.
Salhi, S. and Currie, R. (2009)
Heuristics are here to help your online vehicle scheduling.
OR Insight, 22 (2). pp. 88-104.
Abstract An application of real-time changes in scheduling deliveries of road-making materials is conducted based on an implementation of a tabu search heuristic. This distribution problem deals with heterogeneous products and vehicles where the assignment of pickup points to requests needs also to be made. The problem is investigated as a full-load pickup and delivery problem with time windows. Online as well as offline experiments based on real data from a construction company in the United Kingdom are reported and discussed. Various practical issues that arise in this real-time logistical problem are also discussed and analysed. Interesting and encouraging results are reported.
Wilbaut, C. and Salhi, S. and Hanafi, S. (2009)
An iterative variable-based fixation heuristic for the 0-1 multidimensional knapsack problem.
European Journal of Operational Research (EJOR), 199 (2). pp. 339-348. ISSN 0377-2217.
Abstract An iterative scheme which is based on a dynamic fixation of the variables is developed to solve the 0-1 multidimensional knapsack problem. Such a scheme has the advantage of generating memory information, which is used on the one hand to choose the variables to fix either permanently or temporarily and on the other hand to construct feasible solutions of the problem. Adaptations of this mechanism are proposed to explore different parts of the search space and to enhance the behaviour of the algorithm. Encouraging results are presented when tested on the correlated instances of the 0-1 multidimensional knapsack problem.
Imran, A. and Salhi, S. and Wassan, N.A. (2009)
A variable neighborhood-based heuristic for the heterogeneous fleet vehicle routing problem.
European Journal of Operational Research, 197 (2). pp. 509-518. ISSN 0377-2217.
Abstract The heterogeneous fleet vehicle routing problem is investigated using some adaptations of the variable neighborhood search (VNS). The initial solution is obtained by Dijkstra's algorithm based on a cost network constructed by the sweep algorithm and the 2-opt. Our VNS algorithm uses several neighborhoods which are adapted for this problem. In addition, a number of local search methods together with a diversification procedure are used. Two VNS variants, which differ in the order the diversification and Dijkstra's algorithm are used, are implemented. Both variants appear to be competitive and produce new best results when tested on the data sets from the literature. We also constructed larger data sets for which benchmarking results are provided for future comparison.
Wilbaut, C and Hanafi, S. and Salhi, S. (2008)
A survey of effective heuristics and their application to a variety of knapsack problems.
IMA Journal of Management Mathematics, 19 (3). pp. 227-244. ISSN 1471-6798.
Abstract We present a family of knapsack problems (KPs) while highlighting their particular applications. Though most of the problems are derived from the classical KP, the differences arise in the addition or modification of the constraints or in the way the objective function is defined. Appropriate techniques that were found to be successful in solving these problems are briefly reviewed. Hybrid methods that combine the strengths of different methods such as exact and heuristics are also briefly discussed. Some research avenues that we believe to be useful and challenging are also pointed out.
Brimberg, J. and Hansen, P. and Mladonovic, N. et al. (2008)
A survey of solution methods for the continuous location allocation problem.
International Journal of Operations Research, 5 (1). pp. 1-12. ISSN 0711-2440.
Abstract In this survey, we examine an important class of facility location problems known as the multisource Weber
problem (also referred to as the continuous location-allocation problem). We also show how recent advances in the use of
metaheuristic rules have significantly improved the ability to solve problems of this type. The new solution methods are
discussed for both the well known multisource Weber problem and its counterpart the capacitated case. Research issues
which we believe to be worthwhile exploring in future are also highlighted.
Moin, N.H. and Salhi, S. (2007)
Inventory routing problems: a logistical overview.
Journal of the Operational Research Society, 58 (9). pp. 1185-1194. ISSN 0160-5682.
Abstract This paper presents an overview of Supply Chain Management while focussing on the area of Inventory Routing. We aim to provide the state-of-the-art in this area while highlighting the usefulness of the models in practice as well as their limitations. We have classified the papers based on the planning horizon employed in the models namely single period, multiperiod and infinite horizon models that are then complemented by those with stochastic demand patterns. Future research avenues that we believe to be of interest to the OR/MS community are also presented.
Nagy, G. and Salhi, S. (2007)
Location-routing: Issues, models and methods.
European Journal of Operational Research, 177 (2). pp. 649-672. ISSN 0377-2217.
Abstract This paper is a survey of location-routing: a relatively new branch of locational analysis that takes into account vehicle
routing aspects. We propose a classification scheme and look at a number of problem variants. Both exact and heuristic
algorithms are investigated. Finally, some suggestions for future research are presented.
Salhi, S. and Petch, R. (2007)
A GA based heuristic for the vehicle routing problem with multiple trips.
Journal of Mathematical Modelling and Algorithms, 6 (4). pp. 591-613. ISSN 1572-9214.
Abstract A variant of the classical vehicle routing problem, where vehicles can be assigned to more than one route within a working time period, is investigated. A hybrid Genetic Algorithm, which uses a new non-binary chromosome representation and which is enhanced by a domain specific data structure, appropriate genetic operators and a scheme for chromosome evaluation, is proposed. Test problems from the literature are used to evaluate the performance of the proposed heuristic. Encouraging results are obtained
Zainuddin, Z.M. and Salhi, S. (2007)
A Perturbation-Based Heuristic for the Capacitated Multi-Source Weber Problem.
European Journal of Operational Research, 179 (3). pp. 1194-1207. ISSN 0377-2217.
Abstract This paper proposes a perturbation-based heuristic for the capacitated multisource Weber problem. This procedure is based on an effective use of borderline customers. Several implementations are considered and the two most appropriate are then computationally enhanced by using a reduced neighbourhood when solving the transportation problem. Computational results are presented using data sets from the literature, originally used for the uncapacitated case, with encouraging results.
Welch, S.B. and Salhi, S. and Drezner, Z. (2006)
The Multifacility Maximin Planer Location Problem with Facility Interaction.
IMA Journal of Management Mathematics, 17. pp. 397-412.
Abstract Two branch-and-bound algorithms are proposed to optimally solve the maximin formulation for locating p facilities in the plane. Tight upper and lower bounds are constructed and suitable methods of guiding the search developed. To enhance the method, efficient measures for identifying specific squares for subdivision are suggested. The proposed algorithms are evaluated on a set of randomly generated problems of up to five facilities and 120 nodes.
Drezner, T. and Drezner, Z. and Salhi, S. (2006)
A multi-objective heuristic approach for the casualty collection points location problem.
Journal of the Operational Research Society, 57 (6). pp. 727-734. ISSN 0160-5682.
Abstract In this paper, we formulate the casualty collection points (CCPs) location problem as a multi-objective model. We propose a minimax regret multi-objective (MRMO) formulation that follows the idea of the minimax regret concept in decision analysis. The proposed multi-objective model is to minimize the maximum per cent deviation of individual objectives from their best possible objective function value. This new multi-objective formulation can be used in other multi-objective models as well. Our specific CCP model consists of five objectives. A descent heuristic and a tabu search procedure are proposed for its solution. The procedure is illustrated on Orange County, California.
Salhi, S. and Farber, G. and Coves, A.M. (2006)
Sequencing in a Non-permutation Flowshop with Constrained Buffers: Applicability of Genetic Algorithm versus Constraint Logic Programming.
LNCS, 4818. pp. 536-544.
Abstract Mixed model production lines consider more than one model being processed on the same production line in an arbitrary sequence. Nevertheless, the majority of publications in this area are limited to solutions which determine the job sequence before the jobs enter the line and maintains it without interchanging jobs until the end of the production line, which is known as permutation flowshop. This paper considers a non-permutation flowshop. Resequencing is permitted where stations have access to intermediate or centralized resequencing buffers. The access to the buffers is restricted by the number of available buffer places and the physical size of the products. Two conceptually different approaches are presented in order to solve the problem. The fi rst approach is a hybrid approach, using Constraint Logic Programming (CLP), whereas the second one is a Genetic Algorithm (GA). Improvements that come with the introduction of constrained resequencing buffers are highlighted. Characteristics such as the difference between the intermediate and the centralized case are analyzed,
and the special case of semi dynamic demand is studied. Finally, recommendations are presented for the applicability of the hybrid approach, using CLP, versus the Genetic Algorithm.
Salhi, S. and Brimberg, J. (2005)
A Continuous Location-Allocation Problem With Zone-Dependent Fixed Cost.
Annals of Operations Research, 136 (1). pp. 99-115. ISSN 0254-5330.
Abstract A zone-dependent fixed cost is introduced within the framework of minisum location of facilities in the continuous space. An efficient algorithm for determining the optimal solution for the single facility location problem is put forward, and its properties are validated. A hypothetical example is given to illustrate the algorithm. Some heuristic procedures are proposed for the multi-facility case with encouraging results.
Salhi, S. and Al-Khedhairi, A. (2005)
Enhancements to Two Exact Methods for the Vertex P-Centre Problem.
JMMA, 4 (2). pp. 129-147. ISSN 1570-1166.
Abstract Enhancements to two exact algorithms from the literature to solve the vertex P-center
problem are proposed. In the first approach modifications of some steps are introduced to reduce
the number of ILP iterations needed to find the optimal solution. In the second approach a simple
enhancement which uses tighter initial lower and upper bounds, and a more appropriate binary search
method are proposed to reduce the number of subproblems to be solved. These ideas are tested
on two well known sets of problems from the literature (i.e., OR-Lib and TSP-Lib problems) with
encouraging results
Salhi, S. and Lloyd, L.D. and Johnston, R.L. (2005)
Strategies for Increasing the Efficiency of a Genetic Algorithm for the Structural Optimization of Nanoalloy Clusters.
Journal of Computational Chemistry, 26 (10). pp. 1069-1078. ISSN 0192-8651.
Abstract An improved genetic algorithm (GA) is described that has been developed to increase the efficiency of finding the global minimum energy isomers for nanoalloy clusters. The GA is optimized for the example Pt12Pd12, with specific investigation of: the effect of biasing the initial population by seeding; the effect of removing specified clusters from the population ("predation"); and the effect of varying the type of mutation operator applied. These changes are found to significantly enhance the efficiency of the GA, which is subsequently demonstrated by the application of the best strategy to a new cluster, namely Pt19Pd19. (c) 2005 Wiley Periodicals, Inc.
Nagy, G. and Salhi, S. (2005)
Heuristic Algorithms For Single And Multiple Depot Vehicle Routing Problems With Pickups And Deliveries.
European Journal of Operational Research, 162 (1). pp. 126-141. ISSN 0377-2217.
Abstract The Vehicle Routing Problem with Pickups and Deliveries (VRPPD) is an extension to the classical Vehicle Routing Problem (VRP), where customers may both receive and send goods. We do not make the assumption common in the VRPPD literature, that goods may only be picked up after all deliveries have been completed. We also eschew the concept of insertion and propose a method that treats pickups and deliveries in an integrated manner. This method finds a solution to the corresponding VRP problem and modifies this solution to make it feasible for the VRPPD. Such modification is achieved mainly by heuristic routines taken from VRP methodology but modified such that their aim becomes the reduction of infeasibilities, although a number of problem-specific routines are also constructed. To render our procedures efficient when checking feasibility, we built appropriate mathematical relationships to describe changes in the maximum load of routes. Furthermore, several enhancements are introduced. Our methodology is also capable of solving multi-depot problems, which has not been done before for this challenging general version of the VRPPD. The methods are tested for single and multiple depot problems with encouraging results. (C) 2003 Elsevier B.V. All rights reserved
Salhi, S. and Currie, R. (2004)
A Tabu Search Heuristics for a Full-load Multi-Terminal Vehicle Scheduling Problem with Backhauling and Time Windows.
Journal of Mathematical Modelling and Algorithms, 3 (3). pp. 225-243. ISSN 1572-9214.
Salhi, S. and Smithies, R. and Queen, N.M. (2004)
Adaptive Hybrid Learning for Neural Networks.
Neural Computation, 16 (1). pp. 139-157. ISSN 0899-7667.
Salhi, S. and Mwembeshi, M.M. and Kent, C.A. (2004)
Flexible on-line Modeling and Control of PH in Waste Neutralisation Reactors.
Chemical Engineering & Technology Journal, 27 (2). pp. 130-138. ISSN 0930-7516.
Salhi, S. and Brimberg, J. and Mladonovic, N. (2004)
The Multi-source Weber Problem with Constant Opening.
JORS, 55 (6). pp. 640-646. ISSN 0160-5682.
Salhi, S. and Mwembeshi, M.M and Kent, C.A (2004)
A GA Approach to Robust and Flexible Modelling and Control of PH in Reactors.
Computers & Chemical Engineering, 28 (9). pp. 1743-1757. ISSN 0098-1354.
Salhi, S. and Amponsah, S. (2004)
The investigation of a class of capacitated arc routing problems: the collection of garbage in developing countries.
International Journal of Integrated Waste Management, Science and Technology, 4 (7). pp. 711-721. ISSN 0956-053X.
Salhi, S. and Queen, N.M. (2004)
A Hybrid Algorithm for Identifying Global and Local Minima When Optimizing Functions with Many Minima.
European Journal of Operational Research, 155 (1). pp. 51-67. ISSN 0377-2217.
Salhi, S. and Lloyd, L.D. and Johnston, R.L. et al. (2004)
Theoretical Investigation of Isomer Stability in Platinum-Palladium Nanoallay Clusters.
Journal of Material Chemistry, 14 (11). pp. 1691-1704.
Salhi, S. and Petch, R.J (2003)
A multi-phase constructive heuristic for the vehicle routing problem with multiple trips.
Discrete Applied Mathematics, 133 (1-3). pp. 69-92. ISSN 0166-218X.
Salhi, S. and Gamal, M.D.H. (2003)
A Genetic Algorithm Based Approach for the Uncapacitated Continuous LocationAllocation Problem.
Annals of Operations Research, 123 (1-4). pp. 203-222. ISSN 0254-5330.
Abstract A GA-based approach is introduced to address the continuous locationallocation problem. Selection and removal procedures based on groups of chromosomes instead of individual chromosomes are put forward and specific crossover and mutation operators that rely on the impact of the genes are proposed. A new operator that injects once in a while new chromosomes into the population is also introduced. This provides diversity within the search and attempts to avoid early convergence. This approach is tested on existing data sets using several runs to evaluate the robustness of the proposed GA approach.
Salhi, S. and Currie, R. (2003)
Exact and Heuristic Methods for a Full-Load, Multi-Terminal Vehicle Scheduling Problem with Backhauling and Time Windows.
JORS, 54 (4). pp. 390-400. ISSN 0160-5682.
Abstract The problem considered is the full-load pickup and delivery problem with time windows (PDPTW), and heterogeneous products and vehicles, where the assignment of pickup points to requests is not predetermined. The problem is first formulated as a 0-1 LP, then a hybrid algorithm is developed, which chooses dynamically between a Greedy heuristic and one based on Regret costs. A multi-level constructive heuristic that consists of three post-optimizers is presented. Two lower bounds are used to evaluate the performance of the proposed heuristics when tested on random instances and selected data from a construction company
Salhi, S. and Gamal, M.D.H. (2003)
A cellular heuristic for the multisource Weber problem.
Computers & Operations Research, 30 (11). pp. 1609-1624. ISSN 0305-0548.
Abstract The multisource location-allocation problem in the continuous space is investigated. A learning scheme which uses previous solutions to discretise the continuous space into well-defined cells is proposed. This cells-based technique takes into account frequency of occurrence of already found configurations as well as the compatibility of these configurations. Some results on existing test problems are presented.
Salhi, S. (2002)
Defining Tabu List Size and Aspiration Criterion Within Tabu Search Methods.
Computers & Operations Research, 29 (1). pp. 67-86. ISSN 0305-0548.
Abstract An investigation to explicitly define two key elements in tabu search methods is attempted. In this study a functional representation of the tabu list size is presented and a softer aspiration criterion is put forward. Experiments are conducted on a set of p-median problems.
Salhi, S. and James, C. (2002)
A Tabu Search Heuristic for the Location of Multi-Type Protection Devices on Electrical Supply Tree Networks.
Journal of Combinatorial Optimization, 6 (1). pp. 81-98. ISSN 1382-6905.
Abstract The problem of determining the number of multi-type protection devices and their locations on electrical supply tree networks with subtree dependency is investigated. The aim is to reduce the amount of inconvenience caused to customers that are affected by any given fault on the networks. An appropriate implementation of tabu search is proposed. We exploit a variable neighborhood and a soft aspiration level, and we embed a data structure and reduction tests into the search to speed up the process. Computational tests are performed on randomly generated electrical tree networks varying in size and branch complexity with encouraging results.
Salhi, S. and Drezer, T. and Drezer, Z. (2002)
Solving the multiple competitive facilities location problem.
European Journal of Operational Research, 142 (1). pp. 138-151. ISSN 0377-2217.
Abstract In this paper we propose five heuristic procedures for the solution of the multiple competitive facilities location problem. A franchise of several facilities is to be located in a trade area where competing facilities already exist. The objective is to maximize the market share captured by the franchise as a whole. We perform extensive computational tests and conclude that a two-step heuristic procedure combining simulated annealing and an ascent algorithm provides the best solutions.
Salhi, S. and Wade, A. (2002)
An Investigation into a New Class of Vehicle Routing Problem with Backhauls.
Omega International Journal of Management Science, 30 (6). pp. 479-487. ISSN 0305-0483.
Abstract A new version of the vehicle routing problem with backhauls is presented. In this new problem backhauls are not restricted to be visited once all linehaul customers have been served, neither are backhaul customers fully mixed with linehaul customers. In this problem the user based on his or her experience, the vehicle capacity, the type of products and the type of vehicle used, can define the position along a route from which the first backhaul customer may be visited. An insertion-type heuristic is put forward for this class of problems. An analysis of the improvement in route cost obtained by allowing a relaxation in the restriction of the mix of linehaul and backhaul customers is reported.
Salhi, S. and Drezner, Z. (2002)
Using Hybrid Metaheuristics for the One-Way and Two-Way Network Design Problem.
Naval Research Logistics, 49 (5). pp. 449-463. ISSN 0894-069X.
Abstract A network with traffic between nodes is known. The links of the network can be designed either as two-way links or as one-way links in either direction. The problem is to find the best configuration of the network which minimizes total travel time for all users. Branch and bound optimal algorithms are practical only for small networks (up to 15 nodes). Effective simulated annealing and genetic algorithms are proposed for the solution of larger problems. Both the simulated annealing and the genetic algorithms propose innovative approaches. These innovative ideas can be used in the implementation of these heuristic algorithms for other problems as well. Additional tabu search iterations are applied on the best results obtained by these two procedures. The special genetic algorithm was found to be the best for solving a set of test problems.
Salhi, S. and Gamal, M.D.H. (2001)
Constructive Heuristics for the Uncapacitated Continuous Location-allocation Problem.
Journal of the Operational Research Society, 52 (7). pp. 821-829. ISSN 0160-5682.
Abstract The multisource location-allocation problem in continuous space is investigated: Two constructive heuristic techniques are proposed to solve this problem. Both methods are based on designing suitable schemes for the generation of the initial solutions. The first considers the furthest distance rule and is enhanced by schemes borrowed from tabu search such as constructing the forbidden regions and freeing strategy. The second considers the discrete solutions found when solving the p-median problem. Some results on existing test problems are presented.
Salhi, S. and Thangiah, S.R. (2001)
Genetic Clustering: An Adaptive Heuristic for the Multi Depot Vehicle Routing Problem.
Applied Artificial Intelligence: An International, 15 (4). pp. 361-383. ISSN 1087-6545.
Abstract A generalized clustering method based on a Genetic Algorithm is proposed. The Genetic Clustering (GenClust) method is used for solving the multidepot vehicle routing problem. The solution obtained by the genetic clustering method is improved using an efficient postoptimizer. A set of problems obtained from the literature are used to compare the efficiency of the genetic clustering method for solving the multidepot vehicle routing problem. The genetic clustering method found 11 new best known solutions from the 23 problems in the literature set.
Salhi, S. and Mwembeshi, M.M. and Kent, C.A (2001)
An Approach to Robust and Flexible Modelling and Control of PH in reactors.
Transactions in Chemical Engineering Research and Design, Part A, 79 (A3). pp. 323-334. ISSN 0263-8762.
Abstract Preliminary investigations into the potential application of static feedforward neural networks in the dynamic modelling of pH in complex, time-varying systems have been carried out. To assist in network training and testing, a simplified, 'global first principles (FP) model of the pH of such systems was developed, and used successfully to simulate input output data. Neural networks with input information vectors enhanced by the introduction of auxiliary variables derived from acid-base principles were trained acid tested on this data, using both Levenberg-Marquardt (L-M) and heuristic training algorithms. Both algorithms produced good predictions, but the heuristic algorithm required data pre-treatment to minimize its error. However, it trained much faster than the standard, L-M algorithm.
Salhi, S. and James, C (2000)
The Location of Protection Devices on Electrical Tree Networks: A Heuristic Approach.
JORS, 51 (8). pp. 959-970. ISSN 0160-5682.
Abstract The problem of determining the number of protection devices and their locations on an electrical tree network with subtrees dependency is investigated. The aim is to reduce the amount of inconvenience caused to customers that are affected by any given fault on the network. A constructive heuristic and an appropriate implementation of tabu search are proposed and compared against a method currently used by the electrical supply companies. Computational tests are performed on randomly generated electrical tree networks varying in size and branch complexity. Both the proposed methods outperformed the one used in practice. In particular our tabu search implementation was found to produce the best results without taking an excessive amount of computational time.
Salhi, S. and Drezner, Z. (2000)
Using Tabu Search for Designing One and Two Ways Road Networks.
Control and Cybernetics Journal, 29 (3). pp. 725-740. ISSN 0324-8569.
Salhi, S. and Welch, S.B and Cuningham-Green, R.A (2000)
An Enhancement of an Analytical Approach: The Case of the Weighted Maximum Network Location Problem.
International Journal of Mathematical Algorithms, 1. pp. 315-329. ISSN 1027-9350.
Salhi, S. and Boffey, B. (2000)
Operational Research in Environmental Planning.
European Journal of Operational Research (Special Issue), 121 (2). pp. 217-217. ISSN 0377-2217.
Abstract Environmental considerations are increasingly playing an important part in our lives and many problems are being raised. Operational research has a role to play in solving such problems and this special issue is devoted to the application of operational research methods to environmental planning. For this we were fortunate to receive a variety of high quality papers in the area. From these, eight were finally selected which, with the exception of the invited review, appear in alphabetical order with regard to the first named author.
Charles ReVelle gives a wide ranging review on Research challenges in environmental management'. He concentrates attention on five specific topics: water resources-management of parallel reservoirs; water quality management; solid wastes management; cost allocation; and air quality management. The treatment of each topic starts with background material giving a review of essential achievements to date, followed by some open problems together with some suggestions for possible further research. The issues raised and the problems posed will surely provide a challenge to researchers for many years to come.
The remaining papers can be divided into three categories: theoretical, application and methodological. Of the two papers in the first category, the one by Fernandez et al. describes a general method for the nonconvex problem of locating an undesirable facility' in a plane region based on the use of interval analysis. The other, by Richter and Sombruzki, is concerned with product recovery management as an aid to environmental planning and considers associated production planning and inventory control issues.
Of the application oriented papers, Zhang et al. discuss the contribution that GIS can make in assessing the risk associated with a network link, an essential ingredient for planning the transportation of hazmat (hazardous materials). Reinhard et al. use the SFA and DEA techniques to estimate the environmental efficiency (the ratio of minimum feasible to observed use of environmentally detrimental inputs e.g. nitrogen surplus) of Dutch dairy farms. The third paper in this group details features of a spatial DSS for forest ecosystem management with particular reference to the situation pertaining to the USDA Forest Service.
Another paper concerned with hazmat transportation is that by Akgun et al. The particular aspect they treat is that of finding sets of dissimilar routes with a view to avoiding risk concentration. Finally, Jenkins develops a method to generate a range of scenarios for study in depth in such a way that the maximum amount of useful information can be obtained regarding a possible catastrophic release of pollutant.
It is thus seen that this issue contains an interesting variety of topics related to environmental matters. For this we would like to express our gratitude to all who submitted a paper and to the referees for kindly giving of their time and effort. Finally, we would like to express our thanks to the editors who gave us the opportunity to undertake the preparation of the issue. Monographs
Salhi, S. (2008)
An Application of Real Time Vehicle Scheduling: A Case Study.
working_paper. University of Kent Canterbury, Canterbury
Salhi, S. and Wilbaut, C. and Hanafi, S. (2008)
An Iterative Variable-based Fixation Heuristic for the 0-1 Multidimensional Knapsack Problem.
working_paper. University of Kent Canterbury, Canterbury 10.1016/j.ejor.2008.11.036.
Abstract An iterative scheme which is based on a dynamic fixation of the variables is developed to solve the 0-1 multidimensional knapsack problem. Such a scheme has the advantage of generating memory information, which is used on the one hand to choose the variables to fix either permanently or temporarily and on the other hand to construct feasible solutions of the problem. Adaptations of this mechanism are proposed to explore different parts of the search space and to enhance the behaviour of the algorithm. Encouraging results are presented when tested on the correlated instances of the 0-1 multidimensional knapsack problem.
Salhi, S. (2008)
A Variable Neighborhood-Based Heuristic for the Heterogeneous Fleet Vehicle Routing Problem.
working_paper. University of Kent Canterbury, Canterbury 10.1016/j.ejor.2008.07.022.
Abstract The heterogeneous fleet vehicle routing problem is investigated using some adaptations of the variable neighborhood search (VNS). The initial solution is obtained by Dijkstra's algorithm based on a cost network constructed by the sweep algorithm and the 2-opt. Our VNS algorithm uses several neighborhoods which are adapted for this problem. In addition, a number of local search methods together with a diversification procedure are used. Two VNS variants, which differ in the order the diversification and Dijkstra's algorithm are used, are implemented. Both variants appear to be competitive and produce new best results when tested on the data sets from the literature. We also constructed larger data sets for which benchmarking results are provided for future comparison.
Salhi, S. and Moin, Noor Hasnah (2007)
Inventory Routing Problems: A Logical Overview.
working_paper. Kent Business School, University of Kent, Canterbury
Salhi, S. and Zainuddin, Z.M. (2006)
A Perturbation-Based Heuristic for the Capacitated Multisource Weber Problem.
working_paper. Kent Business School, Canterbury
Abstract This paper proposes a perturbation-based heuristic for the capacitated multisource Weber problem. This procedure is based on an effective use of borderline customers. Several implementations are considered and the two most appropriate are then computationally enhanced by using a reduced neighbourhood when solving the transportation problem. Computational results are presented using data sets from the literature, originally used for the uncapacitated case, with encouraging results.
Nagy, G. and Salhi, S. (2006)
Location-Routing: Issues, Models and Methods.
working_paper. Kent Business School
Salhi, S. and Drezner, T. and Drezner, Z. (2005)
A Multi-Objective Heuristic Approach for the Casualty Collection Points Location Problem.
working_paper. Kent Business School, Canterbury
Abstract In this paper, we formulate the casualty collection points (CCPs) location problem as a multi-objective model. We propose a minimax regret multi-objective (MRMO) formulation that follows the idea of the minimax regret concept in decision analysis. The proposed multi-objective model is to minimize the maximum per cent deviation of individual objectives from their best possible objective function value. This new multi-objective formulation can be used in other multi-objective models as well. Our specific CCP model consists of five objectives. A descent heuristic and a tabu search procedure are proposed for its solution. The procedure is illustrated on Orange County, California.
Nagy, G. and Salhi, S. (2003)
Heuristic Algorithms for Single and Multiple Depot Vehicle Routing Problems with Pickups and Deliveries.
working_paper. Canterbury Business School, Canterbury
Conference Items
Brimberg, J and Elshaikh, A and Mladonovic, N. et al. (2011)
Variable Neighbourhood Search for the Continuous p-centre Problem.
In: 9th Metaheuristics International Conference (MIC2011), 25-28 July 2011, Udine, Italy.
Salhi, S. and Plastino, A. and Fonseca, E. et al. (2009)
Hybrid data mining metaheuristic for the p median problem.
In: UNSPECIFIED SIAM ISBN 978-0-898717-82-5.
Salhi, S. and Llods, L.D. and Johnston, R.L. (2004)
Development of a Genetic Algorithm for Optimization of Nanoalloys.
In: Lectures Notes in Computer Science.
Abstract A genetic algorithm has been developed in order to find the global minimum of platinum-palladium nanoalloy clusters. The effect of biasing the initial population and predating specific clusters has been investigated.
Salhi, S. and Smithie, R. and Queen, N.M. (2003)
Adaptive Learning for Neural Network.
In: 5th International Conference on Metaheuristics, Kyoto.
Nagy, G. and Salhi, S. (2002)
Modelling Considerations in Location-Routing.
In: ECCO-XV, 2002, Lugano. (unpublished)
Salhi, S. and Wade, A. (2001)
An Ant System Algorithm for the Vehicle Routing Problem with Backhauls.
In: 4th International Conference on Metaheuristics, July 16-20, 2001, Porto.
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