Crew Scheduling for Netherlands Railways: "destination: customer"

: In this paper we describe the use of a set covering model with additional constraints for scheduling train drivers and conductors for the Dutch railway operator NS Reizigers. The schedules were generated according to new rules originating from the project "Destination: Customer" ("Bestemming: Klant" in Dutch). This project is carried out by NS Reizigers in order to increase the quality and the punctuality of its train services. With respect to the scheduling of drivers and conductors, this project involves the generation of efficient and acceptable duties with a high robustness against the transfer of delays of trains. A key issue for the acceptability of the duties is the included amount of variation per duty. The applied set covering model is solved by dynamic column generation techniques, Lagrangean relaxation and powerful heuristics. The model and the solution techniques are part of the TURNI system, which is currently used by NS Reizigers for carrying out several analyses concerning the required capacities of the depots. The latter are strongly influenced by the new rules.crew scheduling;dynamic column generation;lagrange relaxation;railways;set covering model

On the knapsack closure of 0-1 integer linear programs

Many inequalities for Mixed-Integer Linear Programs (MILPs) or pure Integer Linear Programs (ILPs) are derived from the Gomory corner relaxation, where all the nonbinding constraints at an optimal LP vertex are relaxed. Computational results show that the corner relaxation gives a good approximation of the integer hull for problems with general-integer variables, but the approximation is less satisfactory for problems with 0-1 variables only. A possible explanation is that, for 0-1 ILPs, even the non-binding variable bound constraints xj≥0 or xj≤1 play an important role, hence their relaxation produces weaker bounds.In this note we address a relaxation for 0-1 ILPs that explicitly takes all variable bound constraints into account. More specifically, we introduce the concept of knapsack closure as a tightening of the classical Chvátal-Gomory (CG) closure. The knapsack closure is obtained as follows: for all inequalities wTx≥w0 valid for the LP relaxation, add to the original system all the valid inequalities for the knapsack polytope conv{xε{0,1}n:wTx≥w0}. A MILP model for the corresponding separation problem is also introduced. © 2010 Elsevier B.V

Direct and Heterodyne Detection of Microwaves in a Metallic Single Wall Carbon Nanotube

This letter reports measurements of microwave (up to 4.5 GHz) detection in metallic single-walled carbon nanotubes. The measured voltage responsivity was found to be 114 V/W at 77K. We also demonstrated heterodyne detection at 1 GHz. The detection mechanism can be explained based on standard microwave detector theory and the nonlinearity of the DC IV-curve. We discuss the possible causes of this nonlinearity. While the frequency response is limited by circuit parasitics in this measurement, we discuss evidence that indicates that the effect is much faster and that applications of carbon nanotubes as terahertz detectors are feasible

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

Bilevel 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

Monte Carlo and hydrodynamic simulation of a one dimensional n+ – n – n+ silicon diode

An improved closure relation - based on the entropy principle - is implemented in a Hydrodynamic model for electron transport. Steady-state electron transport in the "benchmark" n+ - n - n+ submicron silicon diode is simulated and the quality of the model is assessed by comparison with Monte Carlo results

Branching on multi-aggregated variables

open5siopenGamrath, Gerald; Melchiori, Anna; Berthold, Timo; Gleixner, Ambros M.; Salvagnin, DomenicoGamrath, Gerald; Melchiori, Anna; Berthold, Timo; Gleixner, Ambros M.; Salvagnin, Domenic

The role of electron-electron scattering in spin transport

We investigate spin transport in quasi 2DEG formed by III-V semiconductor heterojunctions using the Monte Carlo method. The results obtained with and without electron-electron scattering are compared and appreciable difference between the two is found. The electron-electron scattering leads to suppression of Dyakonov-Perel mechanism (DP) and enhancement of Elliott-Yafet mechanism (EY). Finally, spin transport in InSb and GaAs heterostructures is investigated considering both DP and EY mechanisms. While DP mechanism dominates spin decoherence in GaAs, EY mechanism is found to dominate in high mobility InSb. Our simulations predict a lower spin relaxation/decoherence rate in wide gap semiconductors which is desirable for spin transport.Comment: to appear in Journal of Applied Physic

Generalized phonon-assisted Zener tunneling in indirect semiconductors with non-uniform electric fields : a rigorous approach

A general framework to calculate the Zener current in an indirect semiconductor with an externally applied potential is provided. Assuming a parabolic valence and conduction band dispersion, the semiconductor is in equilibrium in the presence of the external field as long as the electronphonon interaction is absent. The linear response to the electron-phonon interaction results in a non-equilibrium system. The Zener tunneling current is calculated from the number of electrons making the transition from valence to conduction band per unit time. A convenient expression based on the single particle spectral functions is provided, enabling the numerical calculation of the Zener current under any three-dimensional potential profile. For a one dimensional potential profile an analytical expression is obtained for the current in a bulk semiconductor, a semiconductor under uniform field and a semiconductor under a non-uniform field using the WKB (Wentzel-Kramers-Brillouin) approximation. The obtained results agree with the Kane result in the low field limit. A numerical example for abrupt p - n diodes with different doping concentrations is given, from which it can be seen that the uniform field model is a better approximation than the WKB model but a direct numerical treatment is required for low bias conditions.Comment: 29 pages, 7 figure

Characterisation of the secondary-neutron production in particle therapy treatments with the MONDO tracking detector

Particle Therapy (PT) is a non-invasive technique that exploits charged light ions for the irradiation of tumours that cannot be effectively treated with surgery or conventional radiotherapy. While the largest dose fraction is released to the tumour volume by the primary beam, a non-negligible amount of additional dose is due to the beam fragmentation that occurs along the path towards the target volume. In particular, the produced neutrons are particularly dangerous as they can release their energy far away from the treated area, increasing the risk of developing a radiogenic secondary malignant neoplasm after undergoing a treatment. A precise measurement of the neutron flux, energy spectrum and angular distributions is eagerly needed in order to improve the treatment planning system software, so as to predict the normal tissue toxicity in the target region and the risk of late complications in the whole body. The MONDO (MOnitor for Neutron Dose in hadrOntherapy) project is dedicated to the characterisation of the secondary ultra-fast neutrons ([20-400] MeV energy range) produced in PT. The neutron tracking system exploits the reconstruction of the recoil protons produced in two consecutive (n, p) elastic scattering interactions to measure simultaneously the neutron incoming direction and energy. The tracker active media is a matrix of thin squared scintillating fibers arranged in orthogonally oriented layers that are read out by a sensor (SBAM) based on SPAD (Single-Photon Avalanche Diode) detectors developed in collaboration with the Fondazione Bruno Kessler (FBK)