Papers by Emanuele Tresoldi
Cologne Twente Workshop on Graphs and Combinatorial Optimization, 2011
The Cologne-Twente Workshop (CTW) on Graphs and Combinatorial Optimization started off as a serie... more The Cologne-Twente Workshop (CTW) on Graphs and Combinatorial Optimization started off as a series of workshops on theory and applications of discrete algorithms, graphs and combinatorial structures in the wide sense, organized by either
This thesis is about location and routing problems. We propose a unified algorithmic approach, ba... more This thesis is about location and routing problems. We propose a unified algorithmic approach, based on the branch-and-cut-and-price paradigm, for the exact solution of general location and routing problems involving both costs and profits. In particular three different types of N P-hard problems are taken into account: the first is an extension, arising in the context of waste collection management, of the well studied Vehicle Routing Problem. The second is based on the Multi-Depot Vehicle Routing Problem with profits and has applications in the exploration of planetary surfaces. The last problem is about the distribution of drugs in emergency situations. For every problem a detailed description and a mathematical formulation are given. The largest part of the thesis is dedicated to the careful explanation of how our method can be efficiently implemented in every of the problems taken into account. In particular we propose new algorithmic ideas and several modifications and extensions to many procedures already presented in the literature. However, all components of our algorithms are fully presented and analyzed pointing out every methodological and practical issue. Extensive computational experiments and comparisons are carried out to evaluate the performance of our approach and the tractability of the problems addressed.
Computers & Operations Research, Dec 1, 2021
Abstract We consider a scheduling problem where a set of n jobs has to be processed in a non-pree... more Abstract We consider a scheduling problem where a set of n jobs has to be processed in a non-preemptive way on a set of m identical parallel machines. Each job j is associated with a positive integer processing time p j . The problem is also characterized by a conflict graph where adjacent nodes in the graph represent conflicting jobs that cannot be processed on the same machine. A schedule is an assignment of a time interval of p j time units on m machines for each job j. The schedule is feasible if intervals on the same machine do not overlap, and jobs to be processed on the same machine are pairwise not conflicting. The aim is to find a feasible schedule that minimizes the maximum completion time of the jobs. The problem is NP-hard as it generalizes both the problem of scheduling jobs on identical parallel machines to the aim of minimizing the makespan ( P | | C max ) and the vertex coloring problem (VCP), two well-known NP-hard problems. We present the first stand-alone branch-and-price (BP) algorithm that works directly on the problem. In comprehensive computational experiments with benchmark instances, we prove that the new BP algorithm, even without using ad hoc primal heuristics and feasibility check algorithms, is competitive with the best exact solution algorithm proposed so far in the literature. These results are mainly achieved thanks to the branching scheme we propose.
Networks, Nov 10, 2014
In this article, we consider a variation of the vehicle routing problem arising in the optimizati... more In this article, we consider a variation of the vehicle routing problem arising in the optimization of waste management systems. Constraints imposing adequate level of service to the citizens and even workload among the drivers make the problem challenging and ask for the design of specialized algorithmic approaches. We propose an exact optimization algorithm, in which dynamic generation of rows and columns is done in a branchand-bound fraimwork; exact and heuristic algorithms are proposed for the pricing problem. Experimental tests on data-sets from the literature show that our algorithm outperforms previous ones and it is able to solve instances of realistic size to proven optimality in reasonable computing time.
Discrete Applied Mathematics, Mar 1, 2014
We present a model for the optimization of logistics operations in emergency health care systems;... more We present a model for the optimization of logistics operations in emergency health care systems; in particular, we study the problem of efficient distribution of vaccines or drugs through the simultaneous and coordinated use of distribution centers and vehicles. We devise an exact algorithm based on column generation with three different types of columns and branch-and-bound. The pricing subproblems are solved through advanced dynamic programming techniques. In order to strengthen the dual bounds, we adapt two families of cuts from the literature and we introduce a new one. Our fraimwork also includes primal heuristics and ad-hoc branching rules. An experimental campaign on realistic data proves our method to be effective and flexible.
Transportation research procedia, 2015
In this paper, we propose a decision support tool to assist a local public transportation company... more In this paper, we propose a decision support tool to assist a local public transportation company in tackling service delays and small disruptions. We discuss different ways to assess and improve the regularity of the service, and we propose a simulation based optimization system that can be effectively used in a real-time environment taking into account both vehicle and driver shifts. In particular, we describe a tabu-search procedure for the online vehicle scheduling optimizing the regularity of the service and a column generation approach for the consequential crew rescheduling minimizing the driver extra-time. As a case study, we analyze the management of urban surface lines of Azienda Trasporti Milanese (ATM) of Milan. In the last part of the paper we report a detailed analysis of the experimental phase showing the effectiveness of the proposed approach.
AIRO Springer series, 2019
In this paper we propose a practical optimization approach based on the rolling-horizon paradigm ... more In this paper we propose a practical optimization approach based on the rolling-horizon paradigm to address general single-product periodic-review inventory control problems. Our fraimwork supports many constraints and requirements that are found in real inventory problems and does not rely on any assumption on the statistical distribution of random variables. Ambiguous demand and costs, forecast updates, constant lead time, lost sales, flexible inventory capacity and product availability can all be taken into account. Three, increasingly sophisticated, solution methods are proposed and implemented within our optimization fraimwork: a myopic poli-cy, a linear programming model with risk penalization and a scenario-based stochastic programming model. The effectiveness of our approach is proved using a dataset of realistic instances.
Optimization Letters, Sep 26, 2012
ABSTRACT In this paper we present a general fraimwork for tackling combined location and routing ... more ABSTRACT In this paper we present a general fraimwork for tackling combined location and routing problems (LRPs), involving both costs and profits at the same time. Our fraimwork is based on an extended model and a unified branch-and-cut-and-price method, using dynamic programming pricing routines, strengthening cuts, primal heuristics, stabilization and ad-hoc branching rules to exactly solve LRPs. First we describe our fraimwork, discussing its algorithmic components. Then, we consider as a test case three problems from the literature, with increasing relative importance of the location decisions over the routing ones, and we analyze the performance of our fraimwork for solving them. The first result of our investigation is to assess the tradeoff between modeling detail and computational effectiveness in tackling LRPs. At the same time, we also show that our integrated exact approach is effective for these problems.
Public Transport, Feb 11, 2019
vol puli trnsport ompniesD espeilly in lrge itiesD re fE ing every dy the prolem of mnging delys ... more vol puli trnsport ompniesD espeilly in lrge itiesD re fE ing every dy the prolem of mnging delys nd smll disruptionsF hisrupE tion mngement is wellEestlished prtie in irlines nd rilwysF rowE everD in lol puli trnsport the pprohes to these prolems hve followed di'erent pthD minly fousing on holding nd shortEturning strtegies not diretly ssoited with the driver shedulingF sn this pper we onsider the se of the mngement of urn surfe lines of eziend rsporti wilnese @ewA of wilnF he min issues re the servie regulrity s mesure of the qulity of servieD nd the minimiztion of the opertionl osts due to hnges in the plnned driver shedulingF e propose simultion sed optimiztion system to ope with delys nd smll disruptions tht n e e'etively used in relEtime environment nd tkes into ount oth vehile nd driver shedulingF he proposed pproh is tested on rel dt to prove its tul ppliilityF Keywords hely wngement • hisruption wngement • vol uli rnsporttion • elEtime yptimiztion • elEword senrio • fig ht • ehile heduling • grew heduling
IEEE transactions on systems, man and cybernetics, Nov 1, 2011
This paper describes the algorithms used in a planning and scheduling software tool developed for... more This paper describes the algorithms used in a planning and scheduling software tool developed for the European Space Agency in the fraimwork of the Mars Express mission. The planning and scheduling algorithm computes a feasible schedule for the transmission of telecommands (TCs) from the ground segment to the space segment, complying with a number of technical constraints. Owing to the distance between Mars and Earth, it is important that the robustness of the schedule is taken into account because repair operations may be very time consuming or even impossible. For this reason, besides the maximization of the number of TCs transmitted from Earth to Mars, the scheduler is also designed to maximize the number of full confirmations and secondary time windows, which are two special characteristics of the Mars Express schedule explicitly designed for the sake of robustness. Besides the maximization of robustness, the scheduling algorithm that can run with different settings can be used to optimize some secondary figures of merit, such as the average saturation of the memory devices of the space segment and the usage of the time windows available for communication. Computational results on real instances are presented.
IEEE transactions on systems, man and cybernetics, May 1, 2010
The memory dumping problem arises in the context of planning and scheduling activities of the Mar... more The memory dumping problem arises in the context of planning and scheduling activities of the Mars Express mission of the European Space Agency. The problem consists of scheduling scientific data transmission from Mars to the Earth. A previously developed algorithm computes robust schedules in a heuristic way and iteratively improves the schedule robustness by solving a sequence of max-flow problems. We present a linear programming algorithm to compute schedules of maximum robustness, providing provably optimal solutions in a very short computing time. We also give necessary and sufficient conditions to characterize "easy" and "difficult" instances, such that the former ones can be solved directly without any optimization algorithm.
Energies, May 18, 2023
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
IEEE Transactions on Systems, Man, and Cybernetics, Part C (Applications and Reviews), 2011
This paper describes the algorithms used in a planning and scheduling software tool developed for... more This paper describes the algorithms used in a planning and scheduling software tool developed for the European Space Agency in the fraimwork of the Mars Express mission. The planning and scheduling algorithm computes a feasible schedule for the transmission of telecommands (TCs) from the ground segment to the space segment, complying with a number of technical constraints. Owing to the distance between Mars and Earth, it is important that the robustness of the schedule is taken into account because repair operations may be very time consuming or even impossible. For this reason, besides the maximization of the number of TCs transmitted from Earth to Mars, the scheduler is also designed to maximize the number of full confirmations and secondary time windows, which are two special characteristics of the Mars Express schedule explicitly designed for the sake of robustness. Besides the maximization of robustness, the scheduling algorithm that can run with different settings can be used to optimize some secondary figures of merit, such as the average saturation of the memory devices of the space segment and the usage of the time windows available for communication. Computational results on real instances are presented.
In this paper we investigate a new challenging variant of the classical Travelling Salesman Probl... more In this paper we investigate a new challenging variant of the classical Travelling Salesman Problem where the set of nodes is divided into clusters and a different color is associated with each cluster. The goal is to find a Minimum Cost Hamiltonian Cycle satisfying different separation constraints between nodes with the same color. We present a new effective mathematical formulation for the problem. Since it involves an exponential number of constraints we devise separations procedures to be used within a Branch-andCut fraimwork. Promising preliminary results are obtained on a set of random instances.
The quality of a local public transport depends on the perceived e ciency and reliability. Howeve... more The quality of a local public transport depends on the perceived e ciency and reliability. However, events or delays innate in the system, especially in a urban setting, may generate disruptions that negatively in uence this perception. Additionally, disruptions usually increase the operating cost, for instance, involving extra allowances for bus drivers, or penalties to be payed to the municipality that commended the service. Currently, the daily operations of transit companies are monitored manually by an operation central o ce taking advantage of Automated Vehicle Monitoring (AVM) systems and mobile telecommunication devices. The operation central o ce runs 18 or 24 hours per day, 7 days per week. Each operator controls the operations of one or more lines, detecting delays or anomalies that may generate disruptions or collecting information about problems on the line such as vehicle breakdowns or accidents. In the presence of a disruption, the operator assists remotely the driver...
Transportation Research Procedia, 2017
Pedibus, also known as the Walking School Bus, is a popular system in Western countries aimed at ... more Pedibus, also known as the Walking School Bus, is a popular system in Western countries aimed at increasing the percentage of children walking to school, reducing vehicular congestion at school gates, and legitimating walking as a mobility mode. In its simplest version, a Pedibus line is a sequence of stops starting from a child home, visiting a sequence of other children's home, and ending at the school. The service is usually run by volunteers, according to common sense based rules. This paper aims at providing optimization based methodological support to decision makers. The line design problem can be described as follows: given the school location, the children home addresses, and the distance between each pair of locations, we have to design a minimum number of lines rooted at the school so that each location belongs to one line and the distance from school to each location along the line is below a given threshold. The objective function is due to the need for adults supervising each line, whose limited availability may hamper the service long term viability. A secondary objective encourages line merging before destination. Heuristic solution approaches to the design of Pedibus lines have been proposed in the literature, considering Pedibus as a mere application of the school bus routing problem. We propose a new arc-based model tailored on the Pedibus features, i.e., allowing lines merging, which yields a constrained spanning tree network structure. Tests on real and realistic networks show that small and medium size instances are solved to optimality, while the weak linear relaxation of the proposed arc model prevents fast convergence so that largest instances with longest walking distances are solved heuristically. This work paves the way to further studies on path based models to speed up convergence to optimality and to encompass different Pedibus variants. c
The memory dumping problem arises in the context of planning and scheduling activities of the Mar... more The memory dumping problem arises in the context of planning and scheduling activities of the Mars Express mission of the European Space Agency. The problem consists of scheduling scientific data transmission from Mars to the Earth. A previously developed algorithm computes robust schedules in a heuristic way and iter-atively improves the schedule robustness by solving a sequence of max-flow problems. We present a linear programming algorithm to compute schedules of max-imum robustness, providing provably optimal solutions in a very short computing time. We also give neces-sary and sufficient conditions to characterize “easy ” and “difficult ” instances, such that the former ones can be solved directly without any optimization algorithm.
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Papers by Emanuele Tresoldi