Joerg Kalcsics

Several 2-facility location problems on networks with equity objectives

We consider 2-facility location problems with equity measures, defined on networks. The models discussed are, the variance, the mean of absolute weighted deviations, the maximum weighted absolute deviation, the sum of absolute weighted differences, and the range. We give new algorithmic results for these models in the 2-facility case.

On max–min representations of ordered median functions

Abstract An ordered median function is a continuous piecewise linear function. It is well known, that in finite dimensional spaces every continuous piecewise linear function admits a max–min representation in terms of its linear functions. We give an explicit representation of an ordered median function in max–min form using a purely combinatorial approach.

On covering location problems on networks with edge demand

This paper considers two covering location problems on a network where the demand is distributed along the edges. The first is the classical maximal covering location problem. The second problem is the obnoxious version where the coverage should be minimized subject to some distance constraints between the facilities. It is first shown that the finite dominating set for covering problems with nodal demand does not carry over to the case of edge based demands. Then, a solution approach for the single facility problem is presented. Afterwards, the multi-facility problem is discussed and several discretization results for tree networks are presented for the case that the demand is constant on each edge; unfortunately, these results do not carry over to general networks as a counter example shows. To tackle practical problems, the conditional version of the problem is considered and a greedy heuristic is introduced. Afterwards, numerical tests are presented to underline the practicality of the algorithms proposed and to understand the conditions under which accurate modeling of edge-based demand and a continuous edge-based location space are particularly important. HighlightsFirst study of covering location problems on networks with continuously distributed demand along edges.Discretization result for the 1-facility (obnoxious) MCLP where facilities can be located anywhere on the network.Finite dominating sets for the multi-facility (obnoxious) MCLP on trees with constant demand.Finite dominating sets do not carry over to general networks.Efficient solution approach for the conditional problem on general networks with arbitrary demand functions.Sketch of a heuristic solution framework.Numerical comparison with classical demand discretization approaches.

On topological types of ordered median functions

An ordered median functions is a continuous piecewise-linear function. It is well known that in finite dimensional spaces every continuous piecewise-linear function admits a max-min representation in terms of its linear functions. An explicit representation of an ordered median function in max-min form is given by the authors and will appear in a forthcoming issue of this journal. Based on this representation, we give a topological classification of ordered median functions through their simplicial complex of ascent (resp. descent) cones.

Scheduling policies for multi-period services

This paper discusses a multi-period service scheduling problem. In this problem, a set of customers is given who periodically require service over a finite time horizon. To satisfy the service demands, a set of operators is given, each with a fixed capacity in terms of the number of customers an operator can serve per period. The task is to determine for each customer the periods in which he will be visited by an operator such that the periodic service requests of the customers are adhered to and the total number of operators used over the time horizon is minimal. Two alternative policies for scheduling customer visits are considered. In the first one, a customer is visited just on time, i.e., in the period where he or she has a demand for service. The second policy allows service visits ahead of time. The rationale behind this policy is that allowing irregular visits may reduce the overall number of operators needed throughout the time horizon. To solve the problem, integer linear programming formulations are proposed for both policies and numerical experiments are presented that show the reduction in the number of operators used when visits ahead of time are allowed. As only small instances can be solved optimally, a heuristic algorithm is introduced in order to obtain good quality solutions and shorter computing times.

Ascent and descent cones of ordered median block functions

Abstract In this paper, we study properties of a special class of ordered median functions, called block-functions. These are ordered median functions which belong to a generating binary (row)-vector of the form called a block vector. The aim of this paper is to explicitly determine the simplicial complexes and all steepest descent and ascent directions of descent and ascent cones of ordered median block-function.

A branch-and-price algorithm for the Aperiodic Multi-Period Service Scheduling Problem

This paper considers the multi-period service scheduling problem with an aperiodic service policy. In this problem, a set of customers who periodically require service over a finite time horizon is given. To satisfy the service demands, a set of operators is given, each with a fixed capacity in terms of the number of customers that can be served per period. With an aperiodic policy, customers may be served before the period were the service would be due. Two criteria are jointly considered in this problem: the total number of operators, and the total number of ahead-of-time periods. The task is to determine the service periods for each customer in such a way that the service requests of the customers are fulfilled and both criteria are minimized. A new integer programming formulation is proposed, which outperforms an existing formulation. Since the computational effort required to obtain solutions considerably increases with the size of the instances, we also present a reformulation suitable for column generation, which is then integrated within a branch-and-price algorithm. Computational experiments highlight the efficiency of this algorithm for the larger instances.

Structural properties of voronoi diagrams in facility location problems with continuous demand

We consider facility location problems where the demand is continuously and uniformly distributed over a convex polygon with m vertices in the rectilinear plane, n facilities are already present, and the goal is to find an optimal location for an additional facility. Based on an analysis of structural properties of incremental Voronoi diagrams, we develop polynomial exact algorithms for five conditional location problems. The developed methodology is applicable to a variety of other facility location problems with continuous demand. Moreover, we briefly discuss the Euclidean case.