Le problème de l'arborescence de Steiner dans les réseaux tout-optiques

International audienceConnaissant un graphe orienté contenant n noeuds et des arcs pondérés positivement, possédant une racine r et un ensemble X de k noeuds appelés terminaux, le problème de l'arborescence de Steiner (Directed Steiner Tree ou DST) consiste à calculer une arborescence de poids minimal enracinée en r couvrant tous les terminaux. Ce problème est adapté pour modéliser la diffusion multicast dans un réseau quand celui ci ne contient aucun noeud dit non diffusant. Un tel noeud est incapable de copier une donnée qu'il reçoit. Il est donc dans l'obligation pour transférer cette donnée à plusieurs destinataires de la recevoir plusieurs fois. Le poids de son arc entrant est donc démultiplié. La présence de noeuds non diffusants est visible par exemple dans un type de réseaux, dits tout-optiques, où tant que le paquet est encodé sous forme optique, il ne peut être dirigé que vers un seul destinataire. On s'intéresse à une généralisation de DST, nommée Arborescence de Steiner à Branchement Contraint (ASBC) modélisant ce problème de multicast dans le cas d'un réseau où le nombre de noeuds diffusants est limité par un entier d. Nous montrons que ce problème est XP quand il est paramétré par d. Nous montrons également qu'il est possible de construire, à l'aide de ASBC, une |k-1/d|-approximation XP en d pour le problème DST. Enfin, nous montrons que le problème ASBC, sous contrainte que NP n'est pas inclus dans DTIME[n^(O(loglog n))], ne peut être approché polynomialement avec un rapport 1 + (1/e - epsilon) k/(d-1)$, quelque soit epsilon > 0

Inapproximability proof of DSTLB and USTLB in planar graphs

This document proves the problem of finding a minimum cost Steiner Tree covering k terminals with at most p branching nodes (with outdegree greater than 1), in a directed or an undirected planar graph with n nodes, is hard to approximate within a better ratio than n, even when the parameter p is fixed

Vaccine hesitancy: clarifying a theoretical framework for an ambiguous notion.

Today, according to many public health experts, public confidence in vaccines is waning. The term "vaccine hesitancy" (VH) is increasingly used to describe the spread of such vaccine reluctance. But VH is an ambiguous notion and its theoretical background appears uncertain. To clarify this concept, we first review the current definitions of VH in the public health literature and examine its most prominent characteristics. VH has been defined as a set of beliefs, attitudes, or behaviours, or some combination of them, shared by a large and heterogeneous portion of the population and including people who exhibit reluctant conformism (they may either decline a vaccine, delay it or accept it despite their doubts) and vaccine-specific behaviours. Secondly, we underline some of the ambiguities of this notion and argue that it is more a catchall category than a real concept. We also call into question the usefulness of understanding VH as an intermediate position along a continuum ranging from anti-vaccine to pro-vaccine attitudes, and we discuss its qualification as a belief, attitude or behaviour. Thirdly, we propose a theoretical framework, based on previous literature and taking into account some major structural features of contemporary societies, that considers VH as a kind of decision-making process that depends on people's level of commitment to healthism/risk culture and on their level of confidence in the health authorities and mainstream medicine

Adolescent male hazardous drinking and participation in organised activities: Involvement in team sports is associated with less hazardous drinking in young offenders

Aims: The purpose of this research was to test and compare associations between participation in organised activities and indicators of hazardous drinking between young offenders and young non-offenders. Methods: Two groups of 13–18 year-old males were recruited in Cardiff, UK: 93 young offenders and 53 non-offenders from secondary schools matched on estimated IQ, sex and socioeconomic status. Indicators of hazardous drinking were measured using the Fast Alcohol Screening Test (FAST). Organised activity participation and externalising behaviour was measured by the Youth Self Report. The Wechsler Abbreviated Scale of Intelligence was also administered. Results: Young offenders participated in fewer organised activities and had higher FAST scores than non-offenders. Young offenders and non-offenders significantly differed on mean FAST scores if they participated in no organised activities but not if they participated in at least one team sport. Externalising behaviour problems were unrelated to participation in organised activities. Conclusions: Although young offenders were less likely to have participated in organised activities, for them, participation in a team sport was associated with less hazardous drinking. Vulnerable youths who might benefit most from sporting activities actually access them the least. Future research should identify the different barriers to participation that they face

Surface effects in flow boiling of R134a in microtubes

This is the post-print version of the Article. The official published version can be accessed from the link below - Copyright @ 2011 ElsevierThe inner surfaces of microtubes may be influenced strongly by the process of making them due to manufacturing difficulties at these scales compared to larger ones, e.g. the surface characteristics of a seamless cold drawn tube may differ from those of a welded tube. Accordingly, flow boiling heat transfer characteristics may vary. In addition, there is no common agreement between researchers on the criteria of selecting tubes for flow boiling experiments. Instead, tubes are usually ordered from commercial suppliers, in many cases without taking into consideration the manufacturing method and its effect on the heat transfer process. This may explain some of the discrepancies in heat transfer characteristics which are found in the open literature. This paper presents a comparison between experimental flow boiling heat transfer results obtained using two different metallic tubes. The first one is a seamless cold drawn stainless steel tube of 1.1 mm inner diameter while the second is a welded stainless steel tube of 1.16 mm inner diameter. Both tubes have a heated length of 150 mm and the flow direction is vertically upwards. The tubes were heated using DC current. Other experimental conditions include: 8 bar system pressure, 300 kg/m2 s mass flux, about 5K inlet sub-cooling and up to 0.9 exit quality. The results are presented in the form of local heat transfer coefficient versus local quality and axial distance. Also, the boiling curves of the two tubes are discussed. The results show a significant effect of tube inner surface morphology on the heat transfer characteristics

In the Portrait Gallery

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Quality of life assessment in HIV clinical research in resource-limited settings: better late than never.

International audienceComprehensive and sustained optimal care for patients infected with HIV can now be achieved in resource-constrained settings, thanks to the sustainability of programs providing antiretroviral therapy (ART). But the primary goals of HIV virological suppression and improved survival need to be accompanied by a substantial improvement in patient's experience with HIV care and treatment. An assessment of both patients' quality of life and perceived toxicity and symptoms should now be systematically integrated into HIV clinical research in resource-constrained countries. This will allow treatment strategies aimed at optimizing the durability of response to ART in these settings to be properly evaluated and compared

Houses for the Heart to Live In : Finding a Home in the Work of Toi Derricotte

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