Papers by Juan José Salazar González
This paper approaches the problem of designing a two-level network protected against single-edge ... more This paper approaches the problem of designing a two-level network protected against single-edge failures. The problem simultaneously decides on the partition of the set of nodes into terminals and hubs, the connection of the hubs through a backbone network (first network level), and the assignment of terminals to hubs and their connection through access networks (second network level). We consider two survivable structures in both network levels. One structure is a two-edge connected network, and the other structure is a ring. There is a limit on the number of nodes in each access network, and there are fixed costs associated with the hubs and the access and backbone links. The aim of the problem is to minimize the total cost. We give integer programming formulations and valid inequalities for the different versions of the problem, solve them using a branch-and-cut algorithm, and discuss computational results. Some of the new inequalities can be used also to solve other problems in the literature, like the plant cycle location problem and the hub location routing problem.
We address in this article the multi-commodity pickup-and-delivery traveling salesman problem, wh... more We address in this article the multi-commodity pickup-and-delivery traveling salesman problem, which is a routing problem for a capacitated vehicle that has to serve a set of customers that provide or require certain amounts of m different products. Each customer must be visited exactly once by the vehicle, and it is assumed that a unit of a product collected from a customer can be supplied to any other customer that requires that product. Each product is allowed to have several sources and several destinations. The objective is to minimize the total travel distance. We propose a hybrid three-stage heuristic approach that combines a procedure to generate initial solutions with several local search operators and shaking procedures, one of them based on solving an integer programming model. Extensive computational experiments on randomly generated instances with up to 400 locations and 5 products show the effectiveness of the approach.
The sequential ordering problem (SOP) is the generalisation of the asymmetric travelling salesman... more The sequential ordering problem (SOP) is the generalisation of the asymmetric travelling salesman problem in which there are precedence relations between pairs of nodes. Hernández & Salazar introduced a multicommodity flow (MCF) formulation for a generalisation of the SOP in which the vehicle has a limited capacity. We strengthen this MCF formulation by fixing variables and adding valid equations. We then use polyhedral projection, together with some known results on flows, cuts and metrics, to derive new families of strong valid inequalities for both problems. Finally, we give computational results, which show that our findings yield good lower bounds in practice.
Transportation problems are essential in commercial logistics and have been widely studied in the... more Transportation problems are essential in commercial logistics and have been widely studied in the literature during the last decades. Many of them consist in designing routes for vehicles to move commodities between locations. This article approaches a pickup-and-delivery single-vehicle routing problem where there is susceptibility to uncertainty in customer requests. The probability distributions of the requests are assumed to be known, and the objective is to design an a priori route with minimum expected length. The problem has already been approached in the literature, but through a heuristic method. This article proposes the first exact approach to the problem. Two mathematical formulations are proposed: one is a compact model (i.e. defined by a polynomial number of variables and constraints); the other one contains an exponential number of inequalities and is solved within a branch-and-cut framework. Computational results show the upsides as well as the breakdowns of both formulations.
Flight retiming in airline scheduling consists in slightly modifying the scheduled departure time... more Flight retiming in airline scheduling consists in slightly modifying the scheduled departure time of some flights with the goal of providing a better service with a cheaper cost. In this research, the departure times must be selected from a small discrete set of options. The whole problem embeds flight retiming, fleet assignment, aircraft routing and crew pairing. Thus, the aim is to determine the departure times of the flights, the fleet assignment and the minimum cost aircraft and crew routes. The objective function takes into account a large cost associated with each crew member, a penalization for short or long connection times, a cost for crew members changing aircraft along their routes, and a minor penalty associated with the use of each aircraft. The constraints enforce aircraft maintenance and crew working rules. In this setting, flight retiming is allowed to potentially reduce the total costs and increase the robustness of the solution against delays by decreasing the number of aircraft changes.
Lecture Notes in Computer Science, 2004
Abstract Rounding Methods are perturbation techniques used by sta - tistical agencies for protect... more Abstract Rounding Methods are perturbation techniques used by sta - tistical agencies for protecting privacy when publishing aggregated data like frequency tables Controlled rounding is one of these methods that, besides the possibility of publishing clearer rounded tables that still re - spect the additivity underlying the original table, also o ers an e ective and powerful way to protect the unsafe data Due to the lack of au - tomatic procedure, it is at present still not used much, and statistical agencies apply less sophisticated variants (e g , random rounding) which have several disadvantages This work presents an automatic procedure to apply Controlled Rounding in a simple and friendly way on a per - sonal computer The tool is written in C programming language, using Excel SDK - API, and is implemented as an add - in for Microsoft Excel - a standard software we chose due to its wide spread on many personal computers and because of its familiarity to many users who work with tabular material in statistical agencies The algorithm is based on solv - ing an integer linear program in order to find a "good" rounded table while ensuring additivity and protection, as recently proposed by the first author An extra feature of the here - presented implementation is that it does not need to be linked with a commercial mathematical pro - gramming software; better performance on large scale tables would be obtained when Xpress or Cplex is available on the computer
Networks, 2003
CORAL 2003, a Conference on Routing and Location, was held in Puerto de la Cruz (Tenerife, Spain)... more CORAL 2003, a Conference on Routing and Location, was held in Puerto de la Cruz (Tenerife, Spain) from February 24-26, 2003.A wonderful place, close to the black sand of the beach, and a nice temperature welcomed a group of senior and young researchers from Canada, England, France, Germany, and Spain. Social activities were also provided and sponsored by the Cabildo
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics), 2011
This paper presents the main findings when approaching an optimization problem proposed to us by ... more This paper presents the main findings when approaching an optimization problem proposed to us by a telecommunication company in Austria. It concerns deploying a broadband telecommunications system that lays optical fiber cable from a central office to a number of end-customers, i.e., fiber to the home technology. End-customers represent buildings with apartments and/or offices. It is a capacitated network design problem that requires an installation of optical fiber cables with sufficient capacity to carry the traffic from the central office to the endcustomers at minimum cost. This type of problem arises in the design of a Local Access Network (LAN) and in the literature is also named Single-Source Network Loading. In the situation motivating our work the network does not necessarily need to connect all customers (or at least not with the best available technology). Instead, some nodes are potential customers. The aim is to select the customers to be connected to the central server and to choose the cable capacities to establish these connections. The telecom company takes the strategic decision of fixing a percentage of customers that should be served, and aims for minimizing the total cost of the network proving this minimum service. For that reason the underlying problem is called the Prize-Collecting LAN problem (PC-LAN). We propose a sophisticated heuristic to solve real-world instances with up to 86 000 nodes and around 1 500 potential customers.
Omega, 2014
This paper presents the results of a research project funded by a regional carrier operating inte... more This paper presents the results of a research project funded by a regional carrier operating inter-island services within the Canary Islands (Spain) in addition to services to Morocco and Portugal. It operates between 100 and 150 flights a day using three airline operators. The main scope of the project was to solve fleet-assignment, aircraft-routing, crew-pairing and crew-rostering problems on real-world data. The special characteristics of the carrier, flying between 7 am and 11 pm every day, have motivated us to design models and algorithms that are different than the ones addressed in the literature, typically built for large airline companies. This paper shows a solution approach for an integrated fleet-assignment, aircraft-routing and crew-pairing problem covering the flights of a single day. This is a new combinatorial problem that can be considered as a 2-depot vehicle routing problem with driver changes, where the vehicles represent aircrafts and the drivers represent crews. Adapting approaches from the vehicle routing literature, this paper describes a heuristic algorithm based on an integer programming model. In a similar way, this paper also addresses the rostering problem. This problem can be decomposed in smaller problems taking into account operators, bases and crew groups. These problems admit a compact formulation through mixed integer linear programming models which can be tracked by modern general-purpose solvers. This paper illustrates the success of our solution approaches on real-world instances. The airline carrier is currently using these approaches.
Transportation Science, 2013
Lecture Notes in Computer Science, 2010
Computers & Operations Research, 2014
ABSTRACT We study the hub location and routing problem where we decideon the location of hubs, th... more ABSTRACT We study the hub location and routing problem where we decideon the location of hubs, the allocation of nodes to hubs, and the routing among the nodes allocated to the same hubs, with the aim of minimizing the total transportation cost. Each hub has one vehicle that visits all the nodes assigned to it on a cycle. We propose a mixed integer programming formulation for this problem and strengthen it with valid inequalities. We devise separation routines for these inequalities and develop a branch-and-cut algorithm which is tested on CAB and AP instances from the literature. The results show that the formulation is strong and the branch-and-cut algorithm is able to solve instances with up to 50 nodes.
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Papers by Juan José Salazar González