An Approximate Shapley-Folkman Theorem

The Shapley-Folkman theorem shows that Minkowski averages of uniformly bounded sets tend to be convex when the number of terms in the sum becomes much larger than the ambient dimension. In optimization, Aubin and Ekeland [1976] show that this produces an a priori bound on the duality gap of separable nonconvex optimization problems involving finite sums. This bound is highly conservative and depends on unstable quantities, and we relax it in several directions to show that non convexity can have a much milder impact on finite sum minimization problems such as empirical risk minimization and multi-task classification. As a byproduct, we show a new version of Maurey's classical approximate Carath\'eodory lemma where we sample a significant fraction of the coefficients, without replacement, as well as a result on sampling constraints using an approximate Helly theorem, both of independent interest.Comment: Added constraint sampling result, simplified sampling results, reformat, et

A numerical study on active control for tiltrotor whirl flutter stability augmentation

The use of active control to augment whirl flutter stability of tiltrotor aircraft is studied by means of a multibody simulation. The numerical model is based on a 1/5 scale semi-span aeroelastic wind tunnel model of a generic tiltrotor concept and possesses a gimballed, stiff-in-plane rotor that is windmilling. A single-input single-output controller and two types of multi-input multi-output algorithms, Linear Quadratic Gaussian Control and Generalized Predictive Control, are studied. They are using measured wing deflections in order to calculate appropriate swashplate input. Results on the closed-loop behavior of three wing and two gimbal natural modes are given. Robustness analyses with respect to major parameters like wing natural frequencies or structural damping are also briefly discussed. The rotor shear force is shown in the uncontrolled condition and in presence of a controller in order to illustrate the whirl flutter mechanism. The single-input single-output controller yielded substantial gain in stability and turned out to be most suitable for industrial application, whereas the Linear Quadratic Gaussian Regulator yielded even higher damping and still had good robustness characteristics

Choosing security elements for the xAAL home automation system

International audienceThe emergence of Internet of Things (IoT) and smart-home systems allows us to combine devices from different domains and to explore new usages and services. Unfortunately interoperability between devices from different technologies is a major issue to overcome before being able to offer smart services. For this purpose we have proposed the xAAL system. It is both a federating home-automation protocol and an open infrastructure designed to address issues caused by the heterogeneity of existing home-automation solutions. xAAL has been implemented, deployed and has proved its efficiency. However, early versions have been designed with functional concerns in mind. The time has come to address security. xAAL has its own specificities: a distributed system, multicast communications on a bus, etc. This paper details choices, compromises and motivations for selecting security elements that have been introduced in the new version of xAAL

Au-delà des genres de discours : le discours direct à travers les notions de proximité et de communautés de pratique

Le discours rapporté entretient un lien privilégié avec les genres de discours, mais certains phénomènes, notamment la présence de particules d’amorce (introducteurs de discours direct), semblent résister à une approche générique. À partir d’un corpus hétérogène, oral et écrit, illustrant différentes situations de communication (entretiens, conversations ordinaires, forums, conversations instantanées…), nous envisageons les variations de forme et de fonction du discours direct en relation avec le contexte dans lequel il est produit. Ces variations sont dès lors envisagées comme des indices permettant de distinguer des sous-genres de discours. Cependant, cette approche, bien que pertinente, présente des limites pour aborder certaines particules relevées dans les situations d’entretiens. Formuler l’hypothèse que le genre « entretien » motiverait la présence de ces éléments ne peut pas être une explication satisfaisante dans certains cas. Nous envisageons plutôt la relation entre interactants comme un paramètre central dans le choix de certaines unités, qui peut parfois surplomber la question des genres de discours. Nous convoquons alors la notion de communauté de pratique qui permet de concilier le degré de proximité entre interactants et le contexte des productions mais aussi de compléter une approche uniquement générique qui n’est pas toujours suffisante. Si notre propos concerne spécifiquement le discours direct, il pourrait s’appliquer à d’autres formes de discours rapporté, ainsi qu’à d’autres phénomènes linguistiques.Reported speech is closely related to discourse genres, but some phenomena, such as the discourse particles that introduce direct speech, seem to resist the generic approach. Thanks to the analysis of an oral and written heterogeneous corpus, meant to illustrate various communication situations (interviews, ordinary conversations, forums, instant conversations), we look at the variations in the forms and functions of direct speech with respect to the context in which it appears. These variations can be considered as distinctive features of discourse subgenres.However, although this approach is relevant, it is not fully efficient to account for some of the particles to be found in interviews. In some cases, the hypothesis that the presence of these items should be attributed to the interview genre is an insufficient explanation. We rather consider that the relationship between interactants is a central parameter in the choice of some lexemes, and that it is occasionally beyond the scope of discourse genre. We therefore draw on the notion of communities of practice to reconcile the degree of proximity between interactants and the production context. This comes as a supplement to the generic approach, which is not always appropriate. While the analysis deals specifically with direct speech, it could be applied to other forms of reported speech and other linguistic phenomena