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  • Articles  (99)
  • Springer  (99)
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  • 1
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    Resource allocation problems in decentralized energy management (2017)
    Springer
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    Publication Date: 2017-02-14
    Description: Changes in our electricity supply chain are causing a paradigm shift from centralized control towards decentralized energy management. Within the framework of decentralized energy management, devices that offer flexibility in their load profile play an important role. These devices schedule their flexible load profile based on steering signals received from centralized controllers. The problem of finding optimal device schedules based on the received steering signals falls into the framework of resource allocation problems. We study an extension of the traditional problems studied within resource allocation and prove that a divide-and-conquer strategy gives an optimal solution for the considered extension. This leads to an efficient recursive algorithm, with quadratic complexity in the practically relevant case of quadratic objective functions. Furthermore, we study discrete variants of two problems common in decentralized energy management. We show that these problems are NP-hard and formulate natural relaxations of both considered discrete problems that we solve efficiently. Finally, we show that the solutions to the natural relaxations closely resemble solutions to the original, hard problems.
    Print ISSN: 0171-6468
    Electronic ISSN: 1436-6304
    Topics: Mathematics , Economics
    Published by Springer
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  • 2
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    Dynamic network DEA approach with diversification to multi-period performance evaluation of funds (2017)
    Springer
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    Publication Date: 2017-02-22
    Description: When analyzing the relative performance of mutual funds, current data envelopment analysis (DEA) models with diversification ignore the performance change in consecutive periods or the dynamic dependence among investment periods. This paper introduces a novel multi-period network DEA approach with diversification and directional distance function. The new approach decomposes the overall efficiency of a mutual fund in the whole investment interval into efficiencies at individual periods. At each period, mutual funds consume exogenous inputs and intermediate products produced from the preceding period to produce exogenous outputs and intermediate products for the next period to use. Efficiency decomposition reveals the time at which the inefficiency occurs. The new model can provide expected inputs, outputs and intermediate variables at individual periods, which are helpful for managers to find factors causing the overall inefficiency of a fund. Under the assumption of discrete return distributions and a proper choice of inputs, outputs and intermediate variables, the proposed models can be transformed into linear programs. The applicability and reasonability of the proposed method are demonstrated by applying it to assess the relative performance of funds in Chinese security market and European security market, respectively.
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    Electronic ISSN: 1436-6304
    Topics: Mathematics , Economics
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  • 3
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    The benefits of incorporating utility dependencies in finite mixture probit models (2017)
    Springer
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    Publication Date: 2017-05-08
    Description: We propose an application of a new finite mixture multinomial conditional probit (FM-MNCP) model that accommodates preference heterogeneity and explicitly accounts for utility dependencies between choice alternatives considering both local and background contrast effects. The latter is accomplished by using a one-factor structure for segment-specific covariance matrices allowing for nonzero off-diagonal covariance elements. We compare the model to a finite mixture multinomial independent probit (FM-MNIP) model that as well accommodates heterogeneity but assumes independence. That way, we address the potential benefits of a model that additionally accounts for dependencies over a model that accommodates heterogeneity only. Our model comparison is based on empirical data for smoothies and is assessed in terms of fit, holdout validation, and market share predictions. One of the main findings of our empirical study is that allowing for utility dependencies may counterbalance the effects of considering heterogeneity, and vice versa. Additional findings from a simulation study indicate that the FM-MNCP model outperforms the FM-MNIP model with respect to parameter recovery.
    Print ISSN: 0171-6468
    Electronic ISSN: 1436-6304
    Topics: Mathematics , Economics
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    Ergonomic workplace design in the fast pick area (2017)
    Springer
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    Publication Date: 2017-05-05
    Description: The aging society in many developed countries has made an ergonomic workplace design to an important topic among researchers and practitioners alike. We investigate the workplace design for order pickers that manually collect items from the shelves of a warehouse. Specifically, we treat the storage assignment, i.e., the placement of products in shelves of different height, and zoning, i.e., the partitioning of the storage space into areas assigned to separate pickers, in the fast pick area of a warehouse. A fast pick area unifies the most fast-moving items in a compact area, so that workers are relieved from unproductive travel, but face extraordinary ergonomic risks due to the frequent repetition of picking operations. Concerning the health of (aged) workers, it is crucial to reduce such risks. Thus, we define a combined ergonomic storage assignment and zoning problem with the objective of minimizing the maximum ergonomic burden among all workers. This problem is formalized, and two construction heuristics and a tabu search procedure are proposed. Our results show that neglecting ergonomic aspects and only focusing on picking performance leads to much higher ergonomic risks of the workforce.
    Print ISSN: 0171-6468
    Electronic ISSN: 1436-6304
    Topics: Mathematics , Economics
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    Tramp ship routing and scheduling with voyage separation requirements (2017)
    Springer
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    Publication Date: 2017-05-04
    Description: In this paper we explore tramp ship routing and scheduling. Tramp ships operate much like taxies following the available demand. Tramp operators can determine some of their demand in advance by entering into long-term contracts and then try to maximise profits from optional voyages found in the spot market. Routing and scheduling a tramp fleet to best utilise fleet capacity according to current demand is therefore an ongoing and complicated problem. Here we add further complexity to the routing and scheduling problem by incorporating voyage separation requirements that enforce a minimum time spread between some voyages. The incorporation of these separation requirements helps balance the conflicting objectives of maximising profit for the tramp operator and minimising inventory costs for the charterer, since these costs increase if similar voyages are not performed with some separation in time. We have developed a new and exact branch-and-price procedure for this problem. We use a dynamic programming algorithm to generate columns and describe a time window branching scheme used to enforce the voyage separation requirements which we relax in the master problem. Computational results show that our algorithm in general finds optimal solutions very quickly and performs much faster compared to an earlier a priori path generation method. Finally, we compare our method to an earlier adaptive large neighbourhood search heuristic and find that on similar-sized instances our approach generally uses less time to find the optimal solution than the adaptive large neighbourhood search method uses to find a heuristic solution.
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    Electronic ISSN: 1436-6304
    Topics: Mathematics , Economics
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  • 6
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    Integrated planning of spare parts and service engineers with partial backlogging (2017)
    Springer
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    Publication Date: 2017-02-02
    Description: In this paper, we consider the integrated planning of resources in a service maintenance logistics system in which spare parts supply and service engineers deployment are considered simultaneously. The objective is to determine close-to-optimal stock levels as well as the number of service engineers that minimize the total average costs under a maximum total average waiting time constraint. When a failure occurs, a spare part and a service engineer are requested for the repair call. In case of a stock-out at spare parts inventory, the repair call will be satisfied entirely via an emergency channel with a fast replenishment time but at a high cost. However, if the requested spare part is in stock, the backlogging policy is followed for engineers. We model the problem as a queueing network. An exact method and two approximations for the evaluation of a given policy are presented. We exploit evaluation methods in a greedy heuristic procedure to optimize this integrated planning. In a numerical study, we show that for problems with more than five types of spare parts it is preferable to use approximate evaluations as they become significantly faster than exact evaluation. Moreover, approximation errors decrease as problems get larger. Furthermore, we test how the greedy optimization heuristic performs compared to other discrete search algorithms in terms of total costs and computation times. Finally, in a rather large case study, we show that we may incur up to 27% cost savings when using the integrated planning as compared to a separated optimization.
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    Electronic ISSN: 1436-6304
    Topics: Mathematics , Economics
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  • 7
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    Convex cone-based ranking of decision-making units in DEA (2017)
    Springer
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    Publication Date: 2017-04-01
    Description: One of the major research streams in data envelopment analysis (DEA) is ranking decision-making units (DMUs). Utilizing a multicriteria decision-making technique, we develop a novel approach to fully rank all units. Motivated by the convex cone-based total order for multiple criteria alternatives proposed by Dehnokhalaji et al. (Nav Res Logist 61(2):155–163, 2014 ), we consider DMUs in DEA as multiple criteria alternatives and obtain their total ordering. Initially, some pairwise preference information is provided by the decision maker for units and the concepts of convex cones and polyhedral sets are defined in a DEA framework, correspondingly. We apply a modification of Dehnokhalaji et al. method to extract additional preference information for each pair of units and consequently obtain a full ranking (strict total ordering) of DMUs. The benefit of our approach to their method is that we apply non-radial models to overcome the instability drawback of radial models and their infeasibility occurring in DEA applications. The proposed approach is implemented for two numerical examples, and the accuracy of it is investigated through a computational test.
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    Topics: Mathematics , Economics
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  • 8
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    Nash equilibria in location games on a network (2017)
    Springer
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    Publication Date: 2017-01-27
    Description: We study the existence and determination of Nash equilibria (NE) in location games where firms compete for the market with the aim of profit maximization. Each competing firm locates one facility at one point on a network and customers, which are located at the nodes of the network, distribute their buying power between the firms from which they get a minimum price. Two cases are considered depending on price policy: mill pricing and delivered pricing. In the former, the existence of NE depends on the structure of the network and the distribution of demand among its nodes. We give some conditions for the existence of NE, taking into account whether co-location is permitted and whether locations are restricted to nodes. Regarding delivered pricing policy, NE always exist at the nodes if production cost along any edge of the network is concave. A mixed integer linear programming formulation is proposed to find them.
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    Electronic ISSN: 1436-6304
    Topics: Mathematics , Economics
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  • 9
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    Employee substitutability as a tool to improve the robustness in personnel scheduling (2017)
    Springer
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    Publication Date: 2017-02-26
    Description: Organisations usually construct personnel rosters under the assumption of a deterministic operating environment. In the short term, however, organisations operate in a stochastic environment as operational variability arises. This variability leads to the occurrence of unexpected events such as employee absenteeism and/or a demand for personnel that is higher or lower than expected. In order to deal with these uncertainties, organisations need to adopt proactive and reactive scheduling strategies to protect the personnel roster and to respond to this operational variability, respectively. In this paper, we discuss a proactive approach that exploits the concept of employee substitutability to improve the flexibility of a personnel shift roster to respond to schedule disruptions. We propose a pre-emptive programming approach to construct a medium-term personnel shift roster that maximises the employee substitutability value. Moreover, we assess different proactive strategies to introduce robustness with respect to the definition and formulation of employee substitutability and different reactive strategies that impact the decision freedom for schedule recovery. The robustness of the generated personnel shift rosters is evaluated using a three-step methodology of roster construction, daily simulation and optimisation, and evaluation.
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    Topics: Mathematics , Economics
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  • 10
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    Capacity reservation and utilization for a manufacturer with uncertain capacity and demand (2017)
    Springer
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    Publication Date: 2017-01-13
    Description: We consider an original equipment manufacturer (OEM) who has outsourced the production activities to a contract manufacturer (CM). The CM produces for multiple OEMs on the same capacitated production line. The CM requires that all OEMs reserve capacity slots before ordering and responds to these reservations by acceptance or partial rejection, based on allocation rules that are unknown to the OEM. Therefore, the allocated capacity for the OEM is not known in advance, also because the OEM has no information about the reservations of the other OEMs. Based on a real-life situation, we study this problem from the OEM’s perspective who faces stochastic demand and stochastic capacity allocation from the contract manufacturer. We model this problem as a single-item, periodic review inventory system, and we assume linear inventory holding, backorder, and reservation costs. We develop a stochastic dynamic programming model, and we characterize the optimal policy. We conduct a numerical study where we also consider the case that the capacity allocation is dependent on the demand distribution. The results show that the optimal reservation policy is little sensitive to the uncertainty of capacity allocation. In that case, the optimal reservation quantities hardly increase, but the optimal policy suggests increasing the utilization of the allocated capacity. Further, in comparison with a static policy, we show that a dynamic reservation policy is particularly useful when backorder cost and uncertainty are low. Moreover, we show that for the contract manufacturer, to achieve the desired behavior, charging little reservation costs is sufficient.
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    Electronic ISSN: 1436-6304
    Topics: Mathematics , Economics
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