Applications of the Location Set-covering Problem

The location set-covering problem is extended to apply to three new situations. (1) The demands are assumed to occur continuously alone arcs of a network. (2) A mobile unit departs from one of the locations to be chosen and picks up the demand, providing service at a still more distant point. (3) New demands and sites occur over time.     

The Maximal Covering Problem with Some Negative Weights

This article presents the maximal covering problem on a network when some of the weights can be negative. Integer programming formulations are proposed and tested with ILOG CPLEX. Heuristic algorithms, an ascent algorithm, and simulated annealing are proposed and tested. The simulated annealing approach provides the best results for a data set comprising 40 problems.     

Two New Location Covering Problems: The Partial P-Center Problem and the Partial Set Covering Problem

Two new covering problems are introduced. The partial covering P-center problem minimizes a coverage distance in such a way that a given fraction of the population is covered. The partial set covering problem seeks the minimum number of facilities needed to cover an exogenously specified fraction of the population within a given coverage distance. The problems are formulated as integer linear programming problems. Bisection search algorithms are outlined for the two problems. The search algorithm repeatedly solves a Lagrangian relaxation of the maximal covering problem. Computational results for the Lagrangian relaxation of the maximal covering problem and for the bisection search algorithms are presented on problems with up to 150 nodes.     

Two New Location Covering Problems: The Partial P-Center Problem and the Partial Set Covering Problem

Two new covering problems are introduced. The partial covering P-center problem minimizes a coverage distance in such a way that a given fraction of the population is covered. The partial set covering problem seeks the minimum number of facilities needed to cover an exogenously specified fraction of the population within a given coverage distance. The problems are formulated as integer linear programming problems. Bisection search algorithms are outlined for the two problems. The search algorithm repeatedly solves a Lagrangian relaxation of the maximal covering problem. Computational results for the Lagrangian relaxation of the maximal covering problem and for the bisection search algorithms are presented on problems with up to 150 nodes.     

On Covering Problems and the Central Facilities Location Problem

A number of variations of facilities location problems have appeared in the research literature in the past decade. Among these are problems involving the location of multiple new facilities in a discrete solution space, with the new facilities located relative to a set of existing facilities having known locations. In this paper a number of discrete solution space location problems are treated. Specifically, the covering problem and the central facilities location problem are shown to be related. The covering problem involves the location of the minimum number of new facilities among a finite number of sites such that all existing facilities (customers) are covered by at least one new facility. The central facilities location problem consists of the location of a given number of new facilities among a finite number of sites such that the sum of the weighted distances between existing facilities and new facilities is minimized. Computational experience in using the same heuristic solution procedure to solve both problems is provided and compared with other existing solution procedures.     

Computational Comparison of Five Maximal Covering Models for Locating Ambulances

This article categorizes existing maximum coverage optimization models for locating ambulances based on whether the models incorporate uncertainty about (1) ambulance availability and (2) response times. Data from Edmonton, Alberta, Canada are used to test five different models, using the approximate hypercube model to compare solution quality between models. The basic maximum covering model, which ignores these two sources of uncertainty, generates solutions that perform far worse than those generated by more sophisticated models. For a specified number of ambulances, a model that incorporates both sources of uncertainty generates a configuration that covers up to 26% more of the demand than the configuration produced by the basic model.     

Analysis of Errors Due to Demand Data Aggregation in the Set Covering and Maximal Covering Location Problems

In this paper, we extend the concepts of demand data aggregation error to location problems involving coverage. These errors, which arise from losses in locational information, may lead to suboptimal location patterns. They are potentially more significant in covering problems than in p-median problems because the distance metric is binary in covering problems. We examine the Hillsman and Rhoda (1978) Source A, B, and C errors, identify their coverage counterparts, and relate them to the cost and optimality errors that may result. Three rules are then presented which, when applied during data aggregation, will reduce these errors. The third rule will, in fact, eliminate all loss of locational information, but may also limit the amount of aggregation possible. Results of computational tests on a large-scale problem are presented to demonstrate the performance of rule 3.     

Elimination of Source A and B Errors in p-Median Location Problems

The p-median problem is a powerful tool in analyzing facility location options when the goal of the location scheme is to minimize the average distance that demand must traverse to reach its nearest facility. It may be used to determine the number of facilities to site, as well as the actual facility locations. Demand data are frequently aggregated in p-median location problems to reduce the computational complexity of the problem. Demand data aggregation, however, results in the loss of locational information. This loss may lead to suboptimal facility location configurations (optimality errors) and inaccurate measures of the resulting travel distances (cost errors). Hillsman and Rhoda (1978) have identified three error components: Source A, B, and C errors, which may result from demand data aggregation. In this article, a method to measure weighted travel distances in p-median problems which eliminates Source A and B errors is proposed. Test problem results indicate that the proposed measurement scheme yields solutions with lower optimality and cost errors than does the traditional distance measurement scheme.     

The Influence of Data Aggregation on the Stability of p-Median Location Model Solutions

Location planning often makes use of data in an aggregate form without a clear understanding of the consequences. Although research has been directed toward addressing aggregate data usage in location planning, there have been conflicting findings on the stability of location model solutions obtained using aggregated data. This paper analyzes the question of location model solution stability from a somewhat different perspective than previous researchers in that locational configurations identified for aggregate data are evaluated using the original disaggregate data. Analytical results demonstrate that a high level of solution stability does exist when aggregated data are utilized. Further, this analysis is based upon the use of what can be expected to be worst case aggregation approaches. This suggests that the use of aggregate data is adequate for conducting locational studies.     

Probabilistic, Maximal Covering Location—Allocation Models forCongested Systems

When dealing with the design of service networks, such as health andemergency medical services, banking or distributed ticket-selling services, the location of servicecenters has a strong influence on the congestion at each of them, and, consequently, on thequality of service. In this paper, several probabilistic maximal coveringlocation—allocation models with constrained waiting time for queue length are presentedto consider service congestion. The first model considers the location of a given number ofsingle-server centers such that the maximum population is served within a standard distance, andnobody stands in line for longer than a given time or with more than a predetermined number ofother users. Several maximal coverage models are then formulated with one or more servers perservice center. A new heuristic is developed to solve the models and tested in a 30-node network.     

Results of a New Approach to Solving the p-Median Problem with Maximum Distance Constraints

Considerable interest has been directed in the past to developing approaches for solving the p-median problem with maximum distance constraints. All current solution techniques consider potential facilities to be located only at nodes of the network. This paper deals with the solution of this problem under the condition where facility placement is not restricted to nodes. The examples given show that improvement in weighted distance can be obtained by solving the unrestricted site problem. In addition, feasible solutions can be obtained for smaller numbers of facilities than possible by all nodal facility placement.     

Extensions to the Hub Location Problem: Formulations and Numerical Examples

Although a tremendous amount of analytical research is being conducted on the hub location problem, few models exist that extend the number of characteristics found in actual hub-and-spoke networks. Four extensions are presented in this paper: (1) a capacitated network model; (2) a minimum threshold model; (3) a model that endogenously determines the number of open hubs for the network; and (4) a model that incorporates a flow-dependent cost function for the spokes as well as the interhub links. Both the capacitated and the minimum threshold models drop the assumption of a completely interconnected network commonly found in hub location models. Numerical results show that total network costs are often minimized by closing a few interhub links. The third extension is the first known hub location model to determine the optimal number of hubs based on the needs of the network. In this model, the number of open hubs depends on the distribution of flows in the network and how cost effectively the flows can be moved across the network. Previous models that endogenously determined the number of open hubs utilized a fixed cost for establishing each hub in order to limit the number of hubs in the network. The final extension recognizes the potential of all links to amalgamate flows and includes a separate flow-dependent cost function for the spokes in addition to the one for the interhub links. Numerical results are shown for all four models.     

一部承前启后的地方志书——记民国《夏口县志》

民国《夏口县志》是汉口建县后第一部志书,也是汉口建国前唯一一部志书,它全面地反映了清末民初汉口地区的历史与文化,具有很高的史料的地方价值。另外,该志在吸收前人修志经验的同时,纳入进化史观、民生史观以及综合文化观,志书的体例更丰富、更完备,内容更加贴进民众,关注民生、反映民情。使该志初具近代方志的雏形,备中国方志承前启后之功。     

Use of the Mathematical Method in the Solution of a Location Problem

Cost minimization criteria are applied to a study of the optimal location of plants for the manufacture of service-industry machines, including street-cleaning equipment, laundry, and dry-cleaning machines. Future national and regional demand for such machines is estimated. The matrix technique is used to identify the economic region for which the aggregate cost of production and transportation of service-industry machines would be minimized. That region is found to be the Volga-Vyatka region, where the estimated cost of producing 315 million rubles worth of service-industry machines is put at 19.5 million rubles. The author does not necessarily exclude the location of manufacturing plants in other regions, if the purpose is to provide employment, as in areas of depleted mineral deposits of the Urals and the Donets Basin or in the agricultural districts of the Southwest and Central Asia.     

Somalia and Yemen: The Links between Terrorism and State Failure

The scope of this study is to demonstrate that the failed state scheme of Somalia has not been and is not a safe haven for the al‐Qaeda, an issue that has been already displayed in Ken Menkhaus's excellent study (2004 ). Within this context, the article will consider the Somali Islamist political landscape in contrast and in relation to the current situation in Yemen, and thus, it aims to support the premise that weak states, such as Yemen, rather than “collapsed” states, can be incubators of international terrorism. In addition, while studying the interactions between Somalia and Yemen, we will attempt to answer some other important questions arising from this relationship: (1) does the emergence of al‐Qaeda in the Arabian Peninsula and al‐Shabaab have potential convergence; and (2) why is piracy a cross‐border phenomenon and how is it connected to radical Islamism?     

当代留学生与海外华人社会:关于英国诺丁汉华人社会的实证研究

论文以调查问卷作为此次调研的主要手段,以两个既相互联系又有所区别的群体——华人留学生和当地华人居民为研究对象,探讨华人留学生的特点与需求以及华人居民与留学生的共同需求和不同认知。认为留学生不仅是当地华人社会的一个重要组成部分,也是推动华人社会结构调整、组织重构、社会融入等活动的新的动力源泉。留学生与当地社会包括华人社会间存在着交流互动的关系,而不同群体之间存在交往结构及其开放程度的差异,如非大陆留学生比大陆留学生更善于同当地华人居民及非华人团体交往。