Compressed Sensing of Approximately-Sparse Signals: Phase Transitions and Optimal Reconstruction

Compressed sensing is designed to measure sparse signals directly in a compressed form. However, most signals of interest are only "approximately sparse", i.e. even though the signal contains only a small fraction of relevant (large) components the other components are not strictly equal to zero, but are only close to zero. In this paper we model the approximately sparse signal with a Gaussian distribution of small components, and we study its compressed sensing with dense random matrices. We use replica calculations to determine the mean-squared error of the Bayes-optimal reconstruction for such signals, as a function of the variance of the small components, the density of large components and the measurement rate. We then use the G-AMP algorithm and we quantify the region of parameters for which this algorithm achieves optimality (for large systems). Finally, we show that in the region where the GAMP for the homogeneous measurement matrices is not optimal, a special "seeding" design of a spatially-coupled measurement matrix allows to restore optimality.Comment: 8 pages, 10 figure

COLLABORATIVE RESEARCH AND SUSTAINABLE AGRICULTURAL INNOVATION: THE ROLE OF NON ABSORPTIVE INTERMEDIARY ACTORS. THE CASE OF INDUSTRIAL HEMP AND FLAX SECTORS IN FRANCE

N° ISBN - 978-2-7380-1284-5International audienceIndustrial hemp and flax have many uses (paper for hemp, textile for flax, but also fibre incorporation in biocomposites or lime blocks). They are facing great technological expectations with the "Green Future" objectives, both because of sustainable end-uses of hemp and flax and because of their respective sustainable cropping properties. With more than 8000 ha of hemp and 80 000 ha of flax cultivated, France is the biggest European producer. Innovation and hemp valorization is not really taking off (Garnier, Barbier et al., 2007). In order to understand this paradox, this communication proposes to establish a diagnosis, crossing a geographical account and a sociological account of facilities and relationships of innovation based on industrial hemp and flax fibres. From an empirical study on innovation in natural fibres valorization, it aims to stimulate discussion on the specific research approaches for sustainable innovation in agriculture. Among many issues triggered by the technical promise of sustainable innovation, we acknowledge for a lack of specific intermediaries (Howells, 2006) to set up organizational forms and combinative capabilities (Kogut and Zander, 1992) for innovative design. Based on interpretative analysis of the observed dynamics in socio-economic order of Natural Fibres Transformation (Aggeri and Hatchuel, 2003), our communication proposes a scientific discussion of --organizational innovation processes, contrasting with Cohen and Levinthal (1990). This type of actors seems to represent the missing link to entrepreneurships in new configurations of sustainable development

Fablabs : l'institutionnalisation de tiers-lieux du « soft hacking »

We put the movement of creation of many fablabs in a socio-historical perspective by returning to the genesis of the model invented at the MIT and to its replication in France. Accordingly, we present the frameworks and methods of a field study conducted during the expansion of French fablabs during 2013. By limiting our investigation to fablabs that have been opened to the public, we distinguish two categories and we analyse the intermediation activities at work as well as the institutionalisation processes conducted by their founders. In the light of a comparative analysis of these two types of Third-Places, we interpret fablabs as a form of collective experimentation and distributed open innovation that owes much to the trajectories of the founders and to their strategy of "soft hacking" within institutions.Situamos el movimiento de creación de numerosos FabLabs en una perspectiva socio-histórica volviendo a la gestación del modelo inventado en el MIT y a su replicación en Francia. En consecuencia, restauramos marcos y métodos de un estudio sobre el terreno, realizado en el momento de la expansión de FabLabs franceses en 2013. Al limitar nuestra investigación a los FabLabs abiertos al público, distinguimos dos categorías. Analizamos las actividades intermediación dentro de ellos, así como la institucionalización del trabajo realizado por sus fundadores. A la luz de un análisis comparativo de estos dos tipos de terceros lugares en ambas áreas, interpretamos los FabLabs como una forma de experimentación colectiva y distribuida de la innovación abierta que debe mucho a las trayectorias de los fundadores y a su estrategia de "soft hacking" de las instituciones.Nous replaçons le mouvement de création de nombreux fablabs dans une perspective socio-historique en revenant sur la gestation du modèle inventé au MIT et sur sa réplication en France. Partant, nous restituons les cadres et les méthodes d'une étude de terrain conduite au moment de l'expansion des fablabs français en 2013. En limitant notre enquête aux fablabs ouverts au public, nous en distinguons deux catégories. Nous analysons les activités d'intermédiation en leur sein ainsi que le travail d'institutionnalisation réalisé par leurs fondateurs. A l'aune d'une analyse comparée de ces deux types de tiers-lieux, nous interprétons les fablabs comme une forme d'expérimentation collective et distribuée de l'innovation ouverte qui doit beaucoup aux trajectoires des fondateurs et à leur stratégie de « hacking soft » des institutions

Evaluation de la durabilité des systèmes de production agricoles : limites des démarches normatives et voies d'amélioration

International audienceThe concepts of sustainable development and multifunctional agriculture are at the origin of several efforts aiming at their clarification and the development of methods to make them operational. The main principles of agricultural sustainability stress the importance of economic viability, environmental protection and social equity. For farms and farmers, the concept of sustainability is commonly grounded by methods and tools to evaluate their contribution these principles. These methods and tools, abundantly developed in France (IDEA, DIALECTE, ARBRE ...) are commonly based on a fixed set of criteria and indicators chosen a priori, a series of preset measurement and monitoring methods to quantify indicators and aggregation and weighting methods aiming at a single note for the sustainability of the farm. The objective of this contribution is to critically review such types of methods and tools showing their unsuitability for common cases in the context of French agriculture such as (i) farms in transition phases or aiming at radical changes in the farming activities, (ii) multifunctional farms or farms strongly dependent upon off-farm activities and (iii) farms inserted in a territory where their sustainability depends on the spatial and organizational arrangements among different farms. The contribution concludes with some principle to develop evaluation approaches to overcome some of the limitations of existing methods

Dredging Control by Hydrographic Soundings

Where dredging work is to be undertaken, measurement “in situ ” by hydrographic sounding may constitute an excellent basis for payment to the contractor, if the natural sediment movement is negligible compared with the volume to be dredged, and if the thickness of the material to be dredged is too dense to allow for the uncertainty concerning soundings. As a general rule, requirements such as least and maximum depths to be attained, channel model, tolerance, are imposed for dredging areas, and hydrographic soundings are the only available means to assess that these requirements are being met; hence the importance of sounding accuracy. The purpose of this note is to make some comments on this type of measurement by hydrographic sounding and on precautions to be taken in order to ensure its reliability ; these comments appear under three headings :  -measurement of water-depth -position of the survey launch -data processing