Raca Todosijević

Variable neighborhood search: basics and variants

Variable neighborhood search (VNS) is a framework for building heuristics, based upon systematic changes of neighborhoods both in a descent phase, to find a local minimum, and in a perturbation phase to escape from the corresponding valley. In this paper, we present some of VNS basic schemes as well as several VNS variants deduced from these basic schemes. In addition, the paper includes parallel implementations and hybrids with other metaheuristics.

Less is more: Basic variable neighborhood search for minimum differential dispersion problem

Abstract In this paper, we propose a basic variable neighborhood search for solving Minimum differential dispersion problem using only the swap neighborhood structure in both descent (intensification) and shaking (diversification) steps. It has become a trend in the metaheuristic literature to use hybrid metaheuristics, i.e., combination of several metaheuristic paradigms, for solving some particular optimization problem. We show that our simple method, which relies on the basic Variable neighborhood search, significantly outperforms the hybrid one that combines GRASP, Variable neighborhood search, and Exterior path relinking metaheuristics. Thus, simplicity is not only the desired user friendly property of a heuristic but can lead to more efficient and effective method than if complex hybrid metaheuristic is used: less is more.

Two level General variable neighborhood search for Attractive traveling salesman problem

Attractive traveling salesman problem (AtTSP) consists of finding maximal profit tour starting and ending at a given depot after visiting some of the facilities. Total length of the tour must not exceed the given maximum distance. Each facility achieves profit from the customers, based on the distance between the facility and customers as well as on the attractiveness of that facility. Total profit of a tour is equal to a sum of profits of all visited facilities. In this paper, we develop a new variant of Variable neighborhood search, called 2-level General variable neighborhood search (2-GVNS) for solving AtTSP. At the second level, we use General variable neighborhood search in the local search lor building neighboring solution and checking its feasibility. Our 2-GVNS heuristic outperforms tabu search heuristic, the only one proposed in the literature so far, in terms of precision and running times. In addition, 2-GVNS finds all optimal known solutions obtained by Branch and cut algorithm and offers several new best known solutions.

Sequential variable neighborhood descent variants: an empirical study on the traveling salesman problem

Usually several neighborhood structures may be explored within a single local search algorithm. The simplest way is to define a single neighborhood as a union of all predefined neighborhood structures. The another possibility is to make an order (or sequence) of those neighborhoods and use them in the first improvement or the best improvement fashion, following that order. In this work, we firstly classify possible variants of sequential use of neighborhoods and then empirically analyze them in solving the classical travelling salesman problem. Most commonly used TSP neighborhood structures of the same size, such as 2-opt and insertion neighborhoods are explored in our empirical study. We in fact tested 76 different such heuristics on 15,200 random test instances. Several interesting observations are derived. In addition, the two best out of 76 heuristics (used as a local searches within variable neighborhood search (VNS)) are tested on 23 TSPLIB test instances. It appears that union of neighborhoods does not perform well.

Variable neighborhood search for minimum sum-of-squares clustering on networks

Euclidean Minimum Sum-of-Squares Clustering amounts to finding p prototypes by minimizing the sum of the squared Euclidean distances from a set of points to their closest prototype. In recent years related clustering problems have been extensively analyzed under the assumption that the space is a network, and not any more the Euclidean space. This allows one to properly address community detection problems, of significant relevance in diverse phenomena in biological, technological and social systems. However, the problem of minimizing the sum of squared distances on networks have not yet been addressed. Two versions of the problem are possible: either the p prototypes are sought among the set of nodes of the network, or also points along edges are taken into account as possible prototypes. While the first problem is transformed into a classical discrete p-median problem, the latter is new in the literature, and solved in this paper with the Variable Neighborhood Search heuristic. The solutions of the two problems are compared in a series of test examples.

Sum-of-Squares Clustering on Networks

Finding p prototypes by minimizing the sum of the squared distances from a set of points to its closest prototype is a well-studied problem in clustering, data analysis and continuous location. In this note, this very same problem is addressed assuming, for the first time, that the space of possible prototype locations is a network. We develop some interesting properties of such clustering problem. We also show that optimal cluster prototypes are not necessary located at vertices of the network.

A basic variable neighborhood search heuristic for the uncapacitated multiple allocation p-hub center problem

The uncapacitated multiple allocation p-hub center problem (UMApHCP) consists of choosing p hub locations from a set of nodes with pairwise traffic demands in order to route the traffic between the origin-destination pairs such that the maximum cost between origin-destination pairs is minimum. It is assumed that transportation between non-hub nodes is possible only via chosen hub nodes. In this paper we propose a basic variable neighborhood search (VNS) heuristic for solving this NP hard problem. In addition we apply two mathematical formulations of the UMApHCP in order to detect limitations of the current state-of-the-art solver used for this problem. The heuristics are tested on benchmark instances for p-hub problems. The obtained results reveal the superiority of the proposed basic VNS over the state-of-the-art as well as over a multi-start local search heuristic developed by us in this paper.

General variable neighborhood search for the uncapacitated single allocation p-hub center problem

In this paper we propose a general variable neighborhood search heuristic for solving the uncapacitated single allocation p-hub center problem (USApHCP). For the local search step we develop a nested variable neighborhood descent strategy. The proposed approach is tested on benchmark instances from the literature and found to outperform the state-of-the-art heuristic based on ant colony optimization. We also test our heuristic on large scale instances that were not previously considered as test instances for the USApHCP. Moreover, exact solutions were reached by our GVNS for all instances where optimal solutions are known.

Nested general variable neighborhood search for the periodic maintenance problem

In this paper we study the periodic maintenance problem: given a set of m machines and a horizon of T periods, find indefinitely repeating itself maintenance schedule such that at most one machine can be serviced at each period. In addition, all the machines must be serviced at least once for any cycle. In each period the machine i generates a servicing cost bi or an operating cost which depends on the last period in which i was serviced. The operating cost of each machine i in a period equals ai times the number of periods since the last servicing of that machine. The main objective is to find a cyclic maintenance schedule of a periodicity T that minimizes total cost. To solve this problem we propose a new Mixed Integer programming formulation and a new heuristic method based on general Variable neighborhood search called Nested general variable neighborhood search. The performance of this heuristic is shown through an extensive experimentation on a diverse set of problem instances.

Solving the maximum min-sum dispersion by alternating formulations of two different problems

The maximum min-sum dispersion problem aims to maximize the minimum accumulative dispersion among the chosen elements. It is known to be strongly NP-hard problem. In this paper we present heuristic where the objective functions of two different problems are shifted within variable neighborhood search framework. Though this heuristic can be seen as an extended variant of variable formulation search approach that takes into account alternative formulations of one problem, the important difference is that it allows using alternative formulations of more than one optimization problem. Here we use one alternative formulation that is of max-sum type of the originally max–min type maximum diversity problem. Computational experiments on the benchmark instances used in the literature show that the suggested approach improves the best known results for most instances in a shorter computing time.