On the linear stability of vortex columns in the energy space

We investigate the linear stability of inviscid columnar vortices with respect to finite energy perturbations. For a large class of vortex profiles, we show that the linearized evolution group has a sub-exponential growth in time, which means that the associated growth bound is equal to zero. This implies in particular that the spectrum of the linearized operator is entirely contained in the imaginary axis. This contribution complements the results of a previous work, where spectral stability was established for the linearized operator in the enstrophy space.Comment: Major revision, including a complete rewriting of Section

Parallelizing RRT on large-scale distributed-memory architectures

This paper addresses the problem of parallelizing the Rapidly-exploring Random Tree (RRT) algorithm on large-scale distributed-memory architectures, using the Message Passing Interface. We compare three parallel versions of RRT based on classical parallelization schemes. We evaluate them on different motion planning problems and analyze the various factors influencing their performance

Parallelizing RRT on distributed-memory architectures

This paper addresses the problem of improving the performance of the Rapidly-exploring Random Tree (RRT) algorithm by parallelizing it. For scalability reasons we do so on a distributed-memory architecture, using the message-passing paradigm. We present three parallel versions of RRT along with the technicalities involved in their implementation. We also evaluate the algorithms and study how they behave on different motion planning problems

Enhancing the Transition-based RRT to deal with complex cost spaces

The Transition-based RRT (T-RRT) algorithm enables to solve motion planning problems involving configuration spaces over which cost functions are defined, or cost spaces for short. T-RRT has been successfully applied to diverse problems in robotics and structural biology. In this paper, we aim at enhancing T-RRT to solve ever more difficult problems involving larger and more complex cost spaces. We compare several variants of T-RRT by evaluating them on various motion planning problems involving different types of cost functions and different levels of geometrical complexity. First, we explain why applying as such classical extensions of RRT to T-RRT is not helpful, both in a mono-directional and in a bidirectional context. Then, we propose an efficient Bidirectional T-RRT, based on a bidirectional scheme tailored to cost spaces. Finally, we illustrate the new possibilities offered by the Bidirectional T-RRT on an industrial inspection problem

Effects of Long Range Electronic Interactions on a One-dimensional Electron System

The effects of a long range electronic potential on a one dimensional chain of spinless fermions are investigated by numerical techniques (Exact Diagonalisation of rings with up to 30 sites complemented by finite size analysis) and analytic calculations. Due to a competition between the $2 k_F$ oscillations in the density and the (very slow) log divergence of the long wavelength part of the Coulomb potential, the metallic character of the system is enhanced at intermediate (and up to quite large) lengthscales. Despite some similarities, we found that this quasi-metallic regime is not of the Luttinger Liquid type, as evidenced by strong deviations from predictions of conformal field theory and in agreement with the picture of a (very weakly pinned) Wigner crystal. When the strength of the Coulomb interaction is substancially increased, we observe a smooth cross-over to a strongly localized charge density wave.Comment: Latex, Postscripts files include

Disability and social security reforms: The French case

The French pattern of early transitions out of employment is basically explained by the low age at “normal” retirement and by the importance of transitions through unemployment insurance and early-retirement schemes before access to normal retirement. These routes have exempted French workers from massively relying on disability motives for early exits, contrarily to the situation that prevails in some other countries where normal ages are high, unemployment benefits low and early-retirement schemes almost non-existent. Yet the role of disability remains interesting to examine in the French case, at least for prospective reasons in a context of decreasing generosity of other programs. The study of the past reforms of the pension system underlines that disability routes have often acted as a substitute to other retirement routes. Changes in the claiming of invalidity benefits seem to match changes in pension schemes or controls more than changes in such health indicators as the mortality rates. However, our results suggest that increases in average health levels over the past two decades have come along with increased disparities. In that context, less generous pensions may induce an increase in the claiming of invalidity benefits partly because of substitution effects, but also because the share of people with poor health increases.pensions ; social security ; disability ; early retirement ; unemployment ; senior