A New General-Purpose Algorithm for Mixed-Integer Bilevel Linear Programs

International audienceBilevel optimization problems are very challenging optimization models arising in many important practical contexts, including pricing mechanisms in the energy sector, airline and telecommunication industry, transportation networks, critical infrastructure defense, and machine learning. In this paper, we consider bilevel programs with continuous and discrete variables at both levels, with linear objectives and constraints (continuous upper level variables, if any, must not appear in the lower level problem). We propose a general-purpose branch-and-cut exact solution method based on several new classes of valid inequalities, which also exploits a very effective bilevel-specific preprocessing procedure. An extensive computational study is presented to evaluate the performance of various solution methods on a common testbed of more than 800 instances from the literature and 60 randomly generated instances. Our new algorithm consistently outperforms (often by a large margin) alternative state-of-the-art methods from the literature, including methods exploiting problem-specific information for special instance classes. In particular, it solves to optimality more than 300 previously unsolved instances from the literature. To foster research on this challenging topic, our solver is made publicly available online

A MIP-and-refine matheuristic for smart grid energy management

Abstract In the last years we have witnessed an increasing interest in smart buildings, in particular for what concerns optimal energy management, renewable energy sources, and smart appliances. In this paper we investigate the problem of scheduling smart appliance operation in a given time horizon with a set of energy sources and accumulators. Appliance operation is modeled in terms of uninterruptible sequential phases with a given power demand, with the goal of minimizing the energy bill fulfilling duration, energy, and user preference constraints. A Mixed Integer Linear Programming (MIP) model and a greedy heuristic algorithm are given, intended to be used in a synergic way. We show how a general purpose (off-the-shelf) MIP refining procedure can effectively be used for improving, in short computing time, the quality of the solutions provided by the initial greedy heuristic. Computational results confirm the viability of the overall approach, in terms of both solution quality and speed

EFFECTS OF A SPECIFIC TRAINING PROTOCOL ON POSTUROGRAPHIC PARAMETERS OF A TAEKWONDO ELITE ATHLETE AND IMPLICATIONS ON INJURY PREVENTION: A CASE STUDY

Introduction: Taekwondo elite athletes should have excellent qualities in several aspects of fitness, as well as peculiar postural characteristics in terms of balance and postural control, to achieve the best possible results during competitions. Moreover, it is known that taekwondo shows high risk of injury. The aim of this case study is to evaluate the effects of a specific training protocol on posturographic parameters of a taekwondo elite athlete in order to prevent the risk of injury. Materials and methods: The specific training protocol provided 2 months of general strength conditioning, which included pre-conditioning in the first 2 weeks, physical training to improve explosive strength, balance and postural control and athletic training to improve the combat technique. The experimental design included three assessment times: the pre-test (T0), the intermediate- test (T1) and the post-test (T2). Each test provided the same evaluations: anthropometric measurements, cervical ROM assessment and posturographic evaluation. Results: Stabilometric parameters showed a good balance and postural control both in T0 and T2. Baropodometric parameters showed a physiological load pressure distribution between the left and the right foot in T0 (left foot 54% - right foot 46%) as well as in T2 (left foot 45% - right foot 55%). The left forefoot-rearfoot ratio pressure showed no differences between T0 and T2 (p>0.05); we found an improvement on the right forefoot-rearfoot ratio (p<0.05). Cervical range of motion evaluation showed no significant variations from T0 to T2 (p>0.05) on rotation and lateral bending movements; we found an improvement on the extension movement from T0 to T2 (p<0.05), while the flexion movement decreased (p>0.05). Conclusion: The presented case study showed that the experimental protocol improved the postural parameters of this taekwondo elite athlete. We believe that other athletes may adopt this protocol in order to improve own sports performance and to prevent injuries

Reinventing Crew Scheduling at Netherlands Railways

In this paper we describe the successful application of a sophisticated Operations Research model and the corresponding solution techniques for scheduling the 6,500+ drivers and conductors of the Dutch railway operator NS Reizigers (Netherlands Railways). In 2001 the drivers and conductors were very dissatisfied with the structure of their duties, which led to nation wide strikes. However, the application of the model described in this paper led to the development of an alternative production model (‘Sharing Sweet & Sour’) that both satisfied the drivers and conductors, and at the same time supported an increment of the punctuality and efficiency of the railway services. The plans produced according to the alternative production model trimmed personnel costs by about $4.8million (or1.2$7 million per year are also achievable