223 research outputs found
Fabrication de chikwangue au Congo
Ce rapport fait Ă©tat, d'une part, des rĂ©sultats obtenus dans le cadre de recherches entreprises pour dĂ©finir les contextes dans lesquels il est prĂ©vu de diffuser un produit nouveau, la "chiwangue Agricongo", ainsi que les procĂ©dĂ©s et les Ă©quipements mis au point de fabrication et, d'autre part, des actions menĂ©es Ă l'Ă©chelle pilote en vue d'assurer la diffusion de ces innovations. A partir d'enquĂȘtes rĂ©alisĂ©es, Ă Brazzaville et sur toute l'Ă©tendue du territoire, les modalitĂ©s de consommation de la chikwangue au Congo ont Ă©tĂ© dĂ©crites et son importance dans la ration alimentaire des congolais prĂ©cisĂ©e. Les prĂ©fĂ©rences exprimĂ©es et le comportement des consommateurs de chikwangue vis-Ă vis de leur aliment de base ont Ă©tĂ© analysĂ©s. Si la chikwangue reste l'aliment de base prĂ©fĂ©rĂ© des congolais, l'importance de sa consommation varie en fonction de certains facteurs Ă©co-socioĂ©conomiques. D'autres enquĂȘtes rĂ©alisĂ©es dans le mĂȘme temps ont permis d'inventorier et de dĂ©crire les variantes et les innovations endogĂšnes apparues dans les procĂ©dĂ©s de fabrication de la chikwangue en zones rurales et le mode de fonctionnement des ateliers urbains de fabrication. Le rendement des transformations et la durĂ©e et la pĂ©nibilitĂ© des diffĂ©rentes Ă©tapes ont Ă©tĂ© mesurĂ©s. Les procĂ©dĂ©s et les Ă©quipements mis au point Ă partir de 1987 par Agricongo sont dĂ©crits ainsi que, en comparaison avec les systĂšmes traditionnels, le systĂšme de production du groupement de producteurs agricoles dans lequel une ligne de fabrication a Ă©tĂ© installĂ©e en 1991. Les Ă©tudes rĂ©alisĂ©es pour Ă©valuer le rĂ©seau mis en place pour assurer la distribution du produit et l'acceptabilitĂ© des produits, des procĂ©dĂ©s et des Ă©quipements montrent que le produit est trĂšs bien acceptĂ© et que le choix de le commercialiser dans un rĂ©seau de petits commerces est judicieux. Malheureusement, les Ă©quipements se sont rĂ©vĂ©lĂ©s mal adaptĂ©s aux contextes socio-Ă©conomiques, insuffisamment analysĂ©s au moment du dĂ©marrage du projet. D'autre part, le coĂ»t de l'amortissement des machines et la chertĂ© de la matiĂšre premiĂšre en ville ne permet pas d'envisager d'installer des lignes de fabrication en zones urbaines. D'autre part, une fiabilitĂ© encore insuffisante des machines rend alĂ©atoire leur installation prĂšs des zones de production en raison des problĂšmes de maintenance. Par ailleurs, la diffusion des Ă©quipements pris sĂ©parĂ©ment est difficile, compte tenu de leur coĂ»t et du faible niveau habituel d'investissement des ateliers traditionnels. Toutefois, la description rigoureuse des modalitĂ©s de consommation et de transformation de la chikwangue existant actuellement au Congo permet Ă Agricongo, promoteur du projet, de dresser un cahier des charges prĂ©cis pour les amĂ©nagements Ă faire subir aux procĂ©dĂ©s et aux Ă©quipements en vue de les rendre complĂštement adaptĂ©s aux contestes technologiques et Ă©conomiques qui prĂ©valent au Congo. (RĂ©sumĂ© d'auteur
Worldwide spreading of economic crisis
We model the spreading of a crisis by constructing a global economic network
and applying the Susceptible-Infected-Recovered (SIR) epidemic model with a
variable probability of infection. The probability of infection depends on the
strength of economic relations between the pair of countries, and the strength
of the target country. It is expected that a crisis which originates in a large
country, such as the USA, has the potential to spread globally, like the recent
crisis. Surprisingly we show that also countries with much lower GDP, such as
Belgium, are able to initiate a global crisis. Using the {\it k}-shell
decomposition method to quantify the spreading power (of a node), we obtain a
measure of ``centrality'' as a spreader of each country in the economic
network. We thus rank the different countries according to the shell they
belong to, and find the 12 most central countries. These countries are the most
likely to spread a crisis globally. Of these 12 only six are large economies,
while the other six are medium/small ones, a result that could not have been
otherwise anticipated. Furthermore, we use our model to predict the crisis
spreading potential of countries belonging to different shells according to the
crisis magnitude.Comment: 13 pages, 4 figures and Supplementary Materia
A complementary view on the growth of directory trees
Trees are a special sub-class of networks with unique properties, such as the
level distribution which has often been overlooked. We analyse a general tree
growth model proposed by Klemm {\em et. al.} (2005) to explain the growth of
user-generated directory structures in computers. The model has a single
parameter which interpolates between preferential attachment and random
growth. Our analysis results in three contributions: First, we propose a more
efficient estimation method for based on the degree distribution, which is
one specific representation of the model. Next, we introduce the concept of a
level distribution and analytically solve the model for this representation.
This allows for an alternative and independent measure of . We argue that,
to capture real growth processes, the estimations from the degree and the
level distributions should coincide. Thus, we finally apply both
representations to validate the model with synthetically generated tree
structures, as well as with collected data of user directories. In the case of
real directory structures, we show that measured from the level
distribution are incompatible with measured from the degree distribution.
In contrast to this, we find perfect agreement in the case of simulated data.
Thus, we conclude that the model is an incomplete description of the growth of
real directory structures as it fails to reproduce the level distribution. This
insight can be generalised to point out the importance of the level
distribution for modeling tree growth.Comment: 16 pages, 7 figure
Studies of the limit order book around large price changes
We study the dynamics of the limit order book of liquid stocks after
experiencing large intra-day price changes. In the data we find large
variations in several microscopical measures, e.g., the volatility the bid-ask
spread, the bid-ask imbalance, the number of queuing limit orders, the activity
(number and volume) of limit orders placed and canceled, etc. The relaxation of
the quantities is generally very slow that can be described by a power law of
exponent . We introduce a numerical model in order to understand
the empirical results better. We find that with a zero intelligence deposition
model of the order flow the empirical results can be reproduced qualitatively.
This suggests that the slow relaxations might not be results of agents'
strategic behaviour. Studying the difference between the exponents found
empirically and numerically helps us to better identify the role of strategic
behaviour in the phenomena.Comment: 19 pages, 7 figure
Googling the brain: discovering hierarchical and asymmetric network structures, with applications in neuroscience
Hierarchical organisation is a common feature of many directed networks arising in nature and technology. For example, a well-defined message-passing framework based on managerial status typically exists in a business organisation. However, in many real-world networks such patterns of hierarchy are unlikely to be quite so transparent. Due to the nature in which empirical data is collated the nodes will often be ordered so as to obscure any underlying structure. In addition, the possibility of even a small number of links violating any overall âchain of commandâ makes the determination of such structures extremely challenging. Here we address the issue of how to reorder a directed network in order to reveal this type of hierarchy. In doing so we also look at the task of quantifying the level of hierarchy, given a particular node ordering. We look at a variety of approaches. Using ideas from the graph Laplacian literature, we show that a relevant discrete optimization problem leads to a natural hierarchical node ranking. We also show that this ranking arises via a maximum likelihood problem associated with a new range-dependent hierarchical random graph model. This random graph insight allows us to compute a likelihood ratio that quantifies the overall tendency for a given network to be hierarchical. We also develop a generalization of this node ordering algorithm based on the combinatorics of directed walks. In passing, we note that Googleâs PageRank algorithm tackles a closely related problem, and may also be motivated from a combinatoric, walk-counting viewpoint. We illustrate the performance of the resulting algorithms on synthetic network data, and on a real-world network from neuroscience where results may be validated biologically
Structure-guided design and optimization of small molecules targeting the protein-protein interaction between the von hippel-lindau (VHL) E3 ubiquitin ligase and the hypoxia inducible factor (HIF) alpha subunit with in vitro nanomolar affinities
E3 ubiquitin ligases are attractive targets in the ubiquitin-proteasome system, however, the development of small-molecule ligands has been rewarded with limited success. The von Hippel-Lindau protein (pVHL) is the substrate recognition subunit of the VHL E3 ligase that targets HIF-1α for degradation. We recently reported inhibitors of the pVHL:HIF-1α interaction, however they exhibited moderate potency. Herein, we report the design and optimization, guided by X-ray crystal structures, of a ligand series with nanomolar binding affinities
Effective-Range Expansion of the Neutron-Deuteron Scattering Studied by a Quark-Model Nonlocal Gaussian Potential
The S-wave effective range parameters of the neutron-deuteron (nd) scattering
are derived in the Faddeev formalism, using a nonlocal Gaussian potential based
on the quark-model baryon-baryon interaction fss2. The spin-doublet low-energy
eigenphase shift is sufficiently attractive to reproduce predictions by the
AV18 plus Urbana three-nucleon force, yielding the observed value of the
doublet scattering length and the correct differential cross sections below the
deuteron breakup threshold. This conclusion is consistent with the previous
result for the triton binding energy, which is nearly reproduced by fss2
without reinforcing it with the three-nucleon force.Comment: 21 pages, 6 figures and 6 tables, submitted to Prog. Theor. Phy
Unanesthetized Auditory Cortex Exhibits Multiple Codes for Gaps in Cochlear Implant Pulse Trains
Cochlear implant listeners receive auditory stimulation through amplitude-modulated electric pulse trains. Auditory nerve studies in animals demonstrate qualitatively different patterns of firing elicited by low versus high pulse rates, suggesting that stimulus pulse rate might influence the transmission of temporal information through the auditory pathway. We tested in awake guinea pigs the temporal acuity of auditory cortical neurons for gaps in cochlear implant pulse trains. Consistent with results using anesthetized conditions, temporal acuity improved with increasing pulse rates. Unlike the anesthetized condition, however, cortical neurons responded in the awake state to multiple distinct features of the gap-containing pulse trains, with the dominant features varying with stimulus pulse rate. Responses to the onset of the trailing pulse train (Trail-ON) provided the most sensitive gap detection at 1,017 and 4,069Â pulse-per-second (pps) rates, particularly for short (25Â ms) leading pulse trains. In contrast, under conditions of 254Â pps rate and long (200Â ms) leading pulse trains, a sizeable fraction of units demonstrated greater temporal acuity in the form of robust responses to the offsets of the leading pulse train (Lead-OFF). Finally, TONIC responses exhibited decrements in firing rate during gaps, but were rarely the most sensitive feature. Unlike results from anesthetized conditions, temporal acuity of the most sensitive units was nearly as sharp for brief as for long leading bursts. The differences in stimulus coding across pulse rates likely originate from pulse rate-dependent variations in adaptation in the auditory nerve. Two marked differences from responses to acoustic stimulation were: first, Trail-ON responses to 4,069Â pps trains encoded substantially shorter gaps than have been observed with acoustic stimuli; and second, the Lead-OFF gap coding seen for <15Â ms gaps in 254Â pps stimuli is not seen in responses to sounds. The current results may help to explain why moderate pulse rates around 1,000Â pps are favored by many cochlear implant listeners
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