On the Stability Domain of Systems of Three Arbitrary Charges

We present results on the stability of quantum systems consisting of a negative charge $-q_1$ with mass $m_{1}$ and two positive charges $q_2$ and $q_3$, with masses $m_{2}$ and $m_{3}$, respectively. We show that, for given masses $m_{i}$, each instability domain is convex in the plane of the variables $(q_{1}/q_{2}, q_{1}/q_{3})$. A new proof is given of the instability of muonic ions $(\alpha, p, \mu^-)$. We then study stability in some critical regimes where $q_3\ll q_2$: stability is sometimes restricted to large values of some mass ratios; the behaviour of the stability frontier is established to leading order in $q_3/q_2$. Finally we present some conjectures about the shape of the stability domain, both for given masses and varying charges, and for given charges and varying masses.Comment: Latex, 24 pages, 14 figures (some in latex, some in .eps

Organising metabolic networks: cycles in flux distributions

Metabolic networks are among the most widely studied biological systems. The topology and interconnections of metabolic reactions have been well described for many species, but are not sufficient to understand how their activity is regulated in living organisms. The principles directing the dynamic organisation of reaction fluxes remain poorly understood. Cyclic structures are thought to play a central role in the homeostasis of biological systems and in their resilience to a changing environment. In this work, we investigate the role of fluxes of matter cycling in metabolic networks. First, we introduce a methodology for the computation of cyclic and acyclic fluxes in metabolic networks, adapted from an algorithm initially developed to study cyclic fluxes in trophic networks. Subsequently, we apply this methodology to the analysis of three metabolic systems, including the central metabolism of wild type and a deletion mutant of Escherichia coli, erythrocyte metabolism and the central metabolism of the bacterium Methylobacterium extorquens. The role of cycles in driving and maintaining the performance of metabolic functions upon perturbations is unveiled through these examples. This methodology may be used to further investigate the role of cycles in living organisms, their pro-activity and organisational invariance, leading to a better understanding of biological entailment and information processing

Biodiversité aquatique de Madagascar : 15. Les espèces du groupe Simulium cervicornutum (Diptera, Simuliidae)

Les auteurs décrivent, à partir de leur stade nymphal, cinq nouvelles espèces de Simulies du groupe #cervicornutum$ et redécrivent deux espèces connues de ce groupe. (Résumé d'auteur

Nanoscale studies of domain wall motion in epitaxial ferroelectric thin films

Atomic force microscopy was used to investigate ferroelectric switching and nanoscale domain dynamics in epitaxial PbZr0.2Ti0.8O3 thin films. Measurements of the writing time dependence of domain size reveal a two-step process in which nucleation is followed by radial domain growth. During this growth, the domain wall velocity exhibits a v ~ exp[-(1/E)^mu] dependence on the electric field, characteristic of a creep process. The domain wall motion was analyzed both in the context of stochastic nucleation in a periodic potential as well as the canonical creep motion of an elastic manifold in a disorder potential. The dimensionality of the films suggests that disorder is at the origin of the observed domain wall creep. To investigate the effects of changing the disorder in the films, defects were introduced during crystal growth (a-axis inclusions) or by heavy ion irradiation, producing films with planar and columnar defects, respectively. The presence of these defects was found to significantly decrease the creep exponent mu, from 0.62 - 0.69 to 0.38 - 0.5 in the irradiated films and 0.19 - 0.31 in the films containing a-axis inclusions.Comment: 13 pages, 15 figures, to be published in J. App. Phys. special issue on ferroelectric

Two new intermediate polars with a soft X-ray component

Aims. We analyze the first X-ray observations with XMM-Newton of 1RXS J070407.9+262501 and 1RXS 180340.0+401214, in order to characterize their broad-band temporal and spectral properties, also in the UV/optical domain, and to confirm them as intermediate polars. Methods. For both objects, we performed a timing analysis of the X-ray and UV/optical light curves to detect the white dwarf spin pulsations and study their energy dependence. For 1RXS 180340.0+401214 we also analyzed optical spectroscopic data to determine the orbital period. X-ray spectra were analyzed in the 0.2–10.0 keV range to characterize the emission properties of both sources. Results. We find that the X-ray light curves of both systems are energy dependent and are dominated, below 3–5 keV, by strong pulsations at the white dwarf rotational periods (480 s for 1RXS J070407.9+262501 and 1520.5 s for 1RXS 180340.0+401214). In 1RXS 180340.0+401214 we also detect an X-ray beat variability at 1697 s which, together with our new optical spectroscopy, favours an orbital period of 4.4 h that is longer than previously estimated. Both systems show complex spectra with a hard (temperature up to 40 keV) optically thin and a soft (kT ∼ 85–100 eV) optically thick components heavily absorbed by material partially covering the X-ray sources. Conclusions. Our observations confirm the two systems as intermediate polars and also add them as new members of the growing group of “soft” systems which show the presence of a soft X-ray blackbody component. Differences in the temperatures of the blackbodies are qualitatively explained in terms of reprocessing over different sizes of the white dwarf spot. We suggest that systems showing cooler soft X-ray blackbody components also possess white dwarfs irradiated by cyclotron radiation

Comparaison performance-taille des fonds mutuels par une analyse multicritère

L’objet de cette étude est de vérifier si certaines caractéristiques de « risque et de rendement » des fonds mutuels sont clairement reliées à la taille de ces fonds. On se place donc dans un contexte multidimensionnel, où l’on cherche à prendre simultanément en compte les diverses caractéristiques d’un tel placement. Cette vérification est effectuée à l’aide d’une analyse multicritère s’inspirant de la méthode PROMETHEE, laquelle se fonde sur des relations de surclassement.Ce type d’analyse permet également d’expliciter l’impact que peut avoir l’importance relative accordée aux critères, sur la performance des fonds.The purpose of this paper is to verify if some risk-return characteristics of mutual funds are clearly related to their size. Assuming a multidimensional setting where various funds characteristics can be explicitly and simultaneously considered. The paper applies PROMETHEE methodology, based on outranking relations, to make this verification.Such a multicriterion analysis also allows an explicit measure of the impact of the weight given to each criterion on the performance of the funds

Facteurs de risque des actions ordinaires : le point de vue des gestionnaires québécois

L’objet de cet article est d’identifier les principaux facteurs que les gestionnaires professionnels de portefeuilles au Québec prennent en considération pour appréhender le risque de leur placement en actions ordinaires.The purpose of this paper is to identify the main factors professional fund managers in Québec take into consideration when assessing the riskiness of their investment in common stocks