6,861 research outputs found
A new exact algorithm for the multi-depot vehicle routing problem under capacity and route length constraints
This article presents an exact algorithm for the multi-depot vehicle routing problem (MDVRP) under capacity and route length constraints. The MDVRP is formulated using a vehicle-flow and a set-partitioning formulation, both of which are exploited at different stages of the algorithm. The lower bound computed with the vehicle-flow formulation is used to eliminate non-promising edges, thus reducing the complexity of the pricing subproblem used to solve the set-partitioning formulation. Several classes of valid inequalities are added to strengthen both formulations, including a new family of valid inequalities used to forbid cycles of an arbitrary length. To validate our approach, we also consider the capacitated vehicle routing problem (CVRP) as a particular case of the MDVRP, and conduct extensive computational experiments on several instances from the literature to show its effectiveness. The computational results show that the proposed algorithm is competitive against stateof-the-art methods for these two classes of vehicle routing problems, and is able to solve to optimality some previously open instances. Moreover, for the instances that cannot be solved by the proposed algorithm, the final lower bounds prove stronger than those obtained by earlier methods
A Meta-Learning Approach to One-Step Active Learning
We consider the problem of learning when obtaining the training labels is
costly, which is usually tackled in the literature using active-learning
techniques. These approaches provide strategies to choose the examples to label
before or during training. These strategies are usually based on heuristics or
even theoretical measures, but are not learned as they are directly used during
training. We design a model which aims at \textit{learning active-learning
strategies} using a meta-learning setting. More specifically, we consider a
pool-based setting, where the system observes all the examples of the dataset
of a problem and has to choose the subset of examples to label in a single
shot. Experiments show encouraging results
On the Origin of the Type Ia Supernova Width-Luminosity Relation
Brighter Type Ia supernovae (SNe Ia) have broader, more slowly declining
B-band light curves than dimmer SNe Ia. We study the physical origin of this
width-luminosity relation (WLR) using detailed radiative transfer calculations
of Chandrasekhar mass SN Ia models. We find that the luminosity dependence of
the diffusion time (emphasized in previous studies) is in fact of secondary
relevance in understanding the model WLR. Instead, the essential physics
involves the luminosity dependence of the spectroscopic/color evolution of SNe
Ia. Following maximum-light, the SN colors are increasingly affected by the
development of numerous Fe II/Co II lines which blanket the B-band and, at the
same time, increase the emissivity at longer wavelengths. Because dimmer SNe Ia
are generally cooler, they experience an earlier onset of Fe III to Fe II
recombination in the iron-group rich layers of ejecta, resulting in a more
rapid evolution of the SN colors to the red. The faster B-band decline rate of
dimmer SNe Ia thus reflects their faster ionization evolution.Comment: 6 pages, submitted to Ap
Comment on Reply to Comment of Finger et al. (2013) on: 'Evidence for an Early-Middle Miocene age of the Navidad Formation (central Chile): Paleontological, paleoclimatic and tectonic implications' of Gutiérrez et al. (2013, Andean Geology 40 (1): 66-78)
Indexación: Web of Science; ScieloIn their answer to our Comment (Finger et al., 2013), Le Roux et al. (2013) misunderstand several of our remarks and present what we view as f lawed arguments, principally their case for a shallow-marine environment for part of the Navidad Formation. We do not wish to see this exchange evolve into an endless discussion, but we feel obligated to clarify some points. We think this is necessary because of history and importance of the Navidad Formation as the reference for the marine Miocene of Chile. Here we also expound upon some concepts relevant to the distinction between shallow-and deep-marine environments
On the Explosion Mechanism of SNe Type Ia
In this article we discuss the first simulations of two- and
three-dimensional Type Ia supernovae with an improved hydrodynamics code. After
describing the various enhancements, the obtained results are compared to those
of earlier code versions, observational data and the findings of other
researchers in this field.Comment: 7 pages, 4 figure
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