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Saint-Étienne, France

Petit T.,Worcester Polytechnic Institute | De Nantes M.,LINA | Trapp A.C.,Worcester Polytechnic Institute
IJCAI International Joint Conference on Artificial Intelligence | Year: 2015

A number of effective techniques for constraintbased optimization can be used to generate either diverse or high-quality solutions independently, but no framework is devoted to accomplish both simultaneously. In this paper, we tackle this issue with a generic paradigm that can be implemented in most existing solvers. We show that our technique can be specialized to produce diverse solutions of high quality in the context of over-constrained problems. Furthermore, our paradigm allows us to consider diversity from a different point of view, based on generic concepts expressed by global constraints.

Pourchez J.,LINA | Pourchez J.,Ecole Nationale Superieure des Mines de Saint - Etienne CMP | Albuquerque-Silva I.M.D.,LINA | Albuquerque-Silva I.M.D.,Ecole Nationale Superieure des Mines de Saint - Etienne CMP | And 9 more authors.
Journal of Aerosol Science | Year: 2013

New insights on the output of a commercial Technegas generator were proposed in order to optimize the generation of a radioactive nanosized aerosol for human inhalation studies. Parameters influencing Technegas generated aerosols (i.e. gaseous atmosphere, generation temperature and storage duration) were analyzed by a combination of size-fractionation and gamma-scintigraphy detection to determine the aerosol aerodynamic-related distributions. It was found that the total radioactivity per mass and number concentrations of aerosols was mostly influenced by the burn temperature, while the radiolabelling of particles was mostly driven by their surface area. 99mTc labeled nanosized carbonaceous primary particles appear mainly to result from nucleation/condensation of the supersaturated vapor during the burning step, and then coalesce into larger particles due to coagulation processes during the residence time in the expansion chamber. We showed that the burn temperature and the aerosol residence time were the main parameters influencing the particle size distribution. Under optimized operating conditions, the amount of radiolabelled nanoparticles substantially increased since the radioactivity median aerodynamic diameter was reduced by half (250 nm-GSD of 2.5) compared with the standard operating conditions of the Technegas generator (450 nm-GSD of 3.4). © 2012 Elsevier Ltd.

Porumbel D.C.,University of Angers | Hao J.-K.,University of Angers | Kuntz P.,LINA
Computers and Operations Research | Year: 2010

We present a search space analysis and its application in improving local search algorithms for the graph coloring problem. Using a classical distance measure between colorings, we introduce the following clustering hypothesis: the high quality solutions are not randomly scattered in the search space, but rather grouped in clusters within spheres of specific diameter. We first provide intuitive evidence for this hypothesis by presenting a projection of a large set of local minima in the 3D space. An experimental confirmation is also presented: we introduce two algorithms that exploit the hypothesis by guiding an underlying Tabu Search (TS) process. The first algorithm (TS-Div) uses a learning process to guide the basic TS process toward as-yet-unvisited spheres. The second algorithm (TS-Int) makes deep investigations within a bounded region by organizing it as a tree-like structure of connected spheres. We experimentally demonstrate that if such a region contains a global optimum, TS-Int does not fail in eventually finding it. This pair of algorithms significantly outperforms the underlying basic TS algorithm; it can even improve some of the best-known solutions ever reported in the literature (e.g. for dsjc 1000.9). © 2009 Elsevier Ltd. All rights reserved.

Porumbel D.C.,University of Angers | Hao J.-K.,University of Angers | Kuntz P.,LINA
Computers and Operations Research | Year: 2010

We present a diversity-oriented hybrid evolutionary approach for the graph coloring problem. This approach is based on both generally applicable strategies and specifically tailored techniques. Particular attention is paid to ensuring population diversity by carefully controlling spacing among individuals. Using a distance measure between potential solutions, the general population management strategy decides whether an offspring should be accepted in the population, which individual needs to be replaced and when mutation is applied. Furthermore, we introduce a special grouping-based multi-parent crossover operator which relies on several relevant features to identify meaningful building blocks for offspring construction. The proposed approach can be generally characterized as "well-informed", in the sense that the design of each component is based on the most pertinent information which is identified by both experimental observation and careful analysis of the given problem. The resulting algorithm proves to be highly competitive when it is applied on the whole set of the DIMACS benchmark graphs. © 2010 Elsevier Ltd. All rights reserved.

Greffard N.,LINA | Picarougne F.,LINA | Kuntz P.,LINA
Studies in Classification, Data Analysis, and Knowledge Organization | Year: 2012

This paper is focused on the visualization of dynamic social networks, i.e. graphs whose edges model social relationships which evolve during time. In order to overcome the problem of discontinuities of the graphical representations computed by discrete methods, the proposed approach is a continuous one which updates the changes as soon as they happen in the visual restitution. The vast majority of the continuous approaches are restricted to 2D supports which do not optimally match the human perception capabilities. We here present TempoSpring which is a new interactive 3D visualization tool of dynamic graphs. This innovative tool relies on a force-directed layout method to span the 3D space along with several immersive setups (active stereoscopic system/visualization in a dome) to offer an efficient user-experience. TempoSpring has initially been developed in a particular application context: the analysis of sociability networks in the French medieval peasant society. © 2012 Springer-Verlag Berlin Heidelberg.

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