Simultaneous Search and Network Efficiency

When workers send applications to vacancies they create a network. Frictions arise because workers typically do not know where other workers apply to and firms do not know which candidates other firms consider. The first coordination friction affects network formation, while the second coordination friction affects network clearing. We show that those frictions and the wage mechanism are in general not independent. The wage mechanism determines both the distribution of networks that can arise and the number of matches on a given network. Equilibria that exhibit wage dispersion are inefficient in terms of network formation. Under complete recall (firms can go back and forth between all their candidates) only wage mechanisms that allow for ex post Bertrand competition generate the maximum matching on a realized network.random bipartite network formation, network clearing, efficiency, simultaneous search

Simultaneous Search and Network Efficiency

When workers send applications to vacancies they create a network. Frictions arise if workers do not know where other workers apply to (this affects network creation) and firms do not know which candidates other firms consider (this affects network clearing). We show that those frictions and the wage mechanism are in general not independent. Equilibria that exhibit wage dispersion is inefficient in terms of network formation. Under complete recall (firms can go back and forth between all their candidates) only wage mechanisms that allow for ex post Bertrand competition generate the maximum matching on a realized network.efficiency, network clearing, random bipartite network formation, simultaneous search

Quand l'éleveur élève aussi...des arbres. Un esai de protection avec des manchons de jeunes arbres en milieu soudano-sahélien fortement pâturé

International audienceLes éleveurs sont parmi les principaux usagers de l'arbre. Ils s'en servent en particulier pour alimenter leur troupeau en période de soudure, à la fin de la saison sèche. Les pratiques d'exploitation de l'arbre des éleveurs sont parmi les plus élaborées de tous les usagers. Cependant, l'usage durable de cette ressource fourragère nécessite son renouvellement. Celui-ci est fortement lié à la protection dont bénéficie le plant lors de ses premières années. Cette protection est d'autant plus difficile que la pression du bétail est forte, ce qui est le cas notamment autour des campements d'éleveurs, où les systèmes de protections traditionnelles s'avèrent peu efficaces. Cette communication présente les résultats d'une expérimentation de manchons de protection en polypropylène en milieu de forte pression d'élevage, le campement Peul de Macina (200 km de Garoua). Le test a été réalisé sur deux essences, l'Afzelia, producteur de fourrage, et l'Eucalyptus, producteur de bois de service. Il a été doublé en station. L'efficacité du manchon dans les premiers mois de développement du plant, mais aussi l'intérêt des éleveurs pour ce système sont ici discutés. (Résumé d'auteur

Polarisation des flux de bétail par les marchés dans les régions de Ségou et Niono au Mali

48 p.The spatial organization of cattle fluxes in the Malian region of Ségou - Niono features two main facts corresponding to two territorial scales: 1. At first glance the main livestock markets (Ségou and Niono) seem to have a large supply area in common that may be defined relatively to the neighboring regions that do not provide livestock to market; however, back at second reading, this supply basin may be divided in two systems of commercial flux that are well defined, even if they are interdependent: the cattle attractiveness basins of Niono and Ségou. Niono is the big regional cattle market for nomadic or semi-nomadic people coming from the left bank of the Niger River; Ségou is the main cattle market for farmer-herders living in its large periphery in conjunction with the 'Office du Niger'. This distinction clearly appears when considering the length and the orientation of livestock fluxes. The cattle markets attractiveness is still more determined by the livestock systems (pastoralist or agro-herding) than by commercial opportunities.L'organisation spatiale des flux de bovins dans la région de Ségou - Niono présente deux faits majeurs, correspondant à deux échelles territoriales : 1. En première analyse, les marchés principaux, Ségou et Niono, semblent avoir un grand bassin d'approvisionnement en bétail commun qui se définit quasiment en creux des régions pourtant voisines mais non pourvoyeuses de bétail ; 2. en deuxième analyse cependant, ce bassin d'attractivité peut être divisé en deux systèmes de flux commerciaux bien individualisés, même s'ils sont interdépendants : le bassin d'attractivité de Niono et celui de Ségou. Niono est le grand marché régional des éleveurs nomades ou semi-nomades venant des régions pastorales de la rive gauche du fleuve Niger ; tandis que Ségou est le grand marché des agro-éleveurs, vivant dans sa grande périphérie, en lien avec la zone Office du Niger. Cette distinction apparait clairement si on considère la longueur et l'orientation des flux de bétail. Ainsi, l'attractivité des marchés au bétail est toujours davantage déterminée par des systèmes d'élevage (transhumants ou agro-élevage) que par des opportunités d'exporter les animaux vers les marchés de Bamako et ou de l'étranger

Reliable ABC model choice via random forests

Approximate Bayesian computation (ABC) methods provide an elaborate approach to Bayesian inference on complex models, including model choice. Both theoretical arguments and simulation experiments indicate, however, that model posterior probabilities may be poorly evaluated by standard ABC techniques. We propose a novel approach based on a machine learning tool named random forests to conduct selection among the highly complex models covered by ABC algorithms. We thus modify the way Bayesian model selection is both understood and operated, in that we rephrase the inferential goal as a classification problem, first predicting the model that best fits the data with random forests and postponing the approximation of the posterior probability of the predicted MAP for a second stage also relying on random forests. Compared with earlier implementations of ABC model choice, the ABC random forest approach offers several potential improvements: (i) it often has a larger discriminative power among the competing models, (ii) it is more robust against the number and choice of statistics summarizing the data, (iii) the computing effort is drastically reduced (with a gain in computation efficiency of at least fifty), and (iv) it includes an approximation of the posterior probability of the selected model. The call to random forests will undoubtedly extend the range of size of datasets and complexity of models that ABC can handle. We illustrate the power of this novel methodology by analyzing controlled experiments as well as genuine population genetics datasets. The proposed methodologies are implemented in the R package abcrf available on the CRAN.Comment: 39 pages, 15 figures, 6 table

Polychronous mode automata

International audienceAmong related synchronous programming principles, the model of computation of the Polychrony workbench stands out by its capability to give high-level description of systems where each component owns a local activation clock (such as, typically,distributed real-time systems or systems on a chip). In order to bring the modeling capability of Polychrony to the context of a model-driven engineering toolset for embedded system design, we define a diagramic notation composed of mode automata and data-flow equations on top of the multi-clocked synchronous model of computation supported by the Polychrony workbench. We demonstrate the agility of this paradigm by considering the example of an integrated modular avionics application. Our presentation features the formalization and use of model transformation techniques of the GME environment to embed the extension of Polychrony's meta-model with mode automata

Bioinformatic screening of human ESTs for differentially expressed genes in normal and tumor tissues

BACKGROUND: Owing to the explosion of information generated by human genomics, analysis of publicly available databases can help identify potential candidate genes relevant to the cancerous phenotype. The aim of this study was to scan for such genes by whole-genome in silico subtraction using Expressed Sequence Tag (EST) data. METHODS: Genes differentially expressed in normal versus tumor tissues were identified using a computer-based differential display strategy. Bcl-xL, an anti-apoptotic member of the Bcl-2 family, was selected for confirmation by western blot analysis. RESULTS: Our genome-wide expression analysis identified a set of genes whose differential expression may be attributed to the genetic alterations associated with tumor formation and malignant growth. We propose complete lists of genes that may serve as targets for projects seeking novel candidates for cancer diagnosis and therapy. Our validation result showed increased protein levels of Bcl-xL in two different liver cancer specimens compared to normal liver. Notably, our EST-based data mining procedure indicated that most of the changes in gene expression observed in cancer cells corresponded to gene inactivation patterns. Chromosomes and chromosomal regions most frequently associated with aberrant expression changes in cancer libraries were also determined. CONCLUSION: Through the description of several candidates (including genes encoding extracellular matrix and ribosomal components, cytoskeletal proteins, apoptotic regulators, and novel tissue-specific biomarkers), our study illustrates the utility of in silico transcriptomics to identify tumor cell signatures, tumor-related genes and chromosomal regions frequently associated with aberrant expression in cancer