# Obnoxious p-Median Problem

Colmenar, J. M., Greistorfer, P., Martí, R., Duarte A. (2016)

Optsicom project, University of Valencia (Spain)

## Problem Description

Let *I* be a set of clients, *J* a set of facilities, and *d _{ij}* the distance between the client

*i*∈

*I*and the facility

*j*∈

*J*. The Obnoxious

*p*-Median (

*OpM*) problem consists in finding a set

*S*with

*p*facilities (with

*S*⊆

*J*and

*p*< |

*J*|), such that the sum of the minimum distance between each client and the set of facilities is maximized. In mathematical terms:

## State of the Art Methods

The *OpM* problem was introduced in the 1990s (Cappanera, 1999, Eiselt, Laporte, 1995 and Welch, Salhi, 1997) and, as of today, has only gained relatively few attention. It is NP-hard (Tamir, 1991) and can be formulated as a binary LP. Note that Burkard, Fathali, and Taghizadeh Kakhki (2007) proved that the special case of the *OpM* problem on a tree can be solved in linear time. A branch and cut method coupled with a tabu search has been recently suggested by Belotti, Labbé, Maffioli, and Ndiaye (2007).

## Instances

We have used a set of 144 instances in our experimentation. In particular, they were generated by considering 24 instances of the well-known *p*-median problem. To this aim we considered the instances from pmed17 to pmed40, where the number of nodes ranges from 400 to 900. These *p*-median instances are found in the OR-Library (Beasley, J. J Oper Res Soc (1990) 41: 1069. doi:10.1057/jors.1990.166).

In order to transform a *p*-median instance into an obnoxious *p*-median one, Belotti et al. (2007) described the following procedure. Given the original instance with *n* nodes, this method selects *n*/2 nodes at random to be the set of clients. The remaining *n*/2 become the set of facilities. Additionally, for each original instance, Belotti et al. (2007) derived three new instances for the OpM by considering three values of *p*: ⌊*n*/2⌋, ⌊*n*/4⌋ and ⌊*n*/8⌋.

We folowed that process to generate the instances and then, for each one of the resulting files, we generated an *A* and a *B* version of them by transposing the table that stores the distances between clients and facilities. This is a simple way of obtaining two different instances with similar charasteristics for each one of the processed files.

Hence, we stored the resulting 144 instances as a library that we called OpM_LIB:

## Computational Experience

All the algorithms were implemented in Java SE 7 and the experiments were conducted on an Intel Core i5 660 CPU (3.3 GHz) and 8 GB RAM. We have considered the OpM_LIB instances described above. The individual results for each instance can be downloaded here in Excel format.

Obnoxious *p*-Median problem results

## References

- J. E. Beasley. OR-Library: Distributing test problems by electronic mail. Journal of the Operational Research Society, 41 (11) (1990), pp. 1069–1072.
- P. Belotti, M. Labbé, F. Maffioli, M. Ndiaye. A branch-and-cut method for the obnoxious p-median problem.
*4OR*, 5 (4) (2007), pp. 299–314. - R. Burkard, J. Fathali, H. Taghizadeh Kakhki. The p-maxian problem on a tree. Operations Research Letters, 35 (3) (2007), pp. 331–335.
- P. Cappanera. A Survey on obnoxious facility location problems. Technical report Dipartimento di Informatica, Università di Pisa (1999).
- H. Eiselt, G. Laporte. Objectives in location problems. Z. Drezner (Ed.), Facility location. a survey of applications and methods., Springer, New York (1995), pp. 151–180.
- A. Tamir. Obnoxious facility location on graphs. SIAM Journal on Discrete Mathematics, 2 (4) (1991), pp. 550–567.
- S. Welch, S. Salhi. The obnoxious P facility network location problem with facility interaction. European Journal of Operational Research, 102 (1997), pp. 302–319.