145 research outputs found
Evolution entre 2007 et 2009 de la résistance de Pseudomonas aeruginosa à la ciprofloxacine en fonction de la consommation des fluoroquinolones au CHU d'Angers
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Frequency-dependent (ac) Conduction in Disordered Composites: a Percolative Study
In a recent paper [Phys. Rev. B{\bf57}, 3375 (1998)], we examined in detail
the nonlinear (electrical) dc response of a random resistor cum tunneling bond
network (, introduced by us elsewhere to explain nonlinear response of
metal-insulator type mixtures). In this work which is a sequel to that paper,
we consider the ac response of the -based correlated () model.
Numerical solutions of the Kirchoff's laws for the model give a power-law
exponent (= 0.7 near ) of the modulus of the complex ac conductance at
moderately low frequencies, in conformity with experiments on various types of
disordered systems. But, at very low frequencies, it gives a simple quadratic
or linear dependence on the frequency depending upon whether the system is
percolating or not. We do also discuss the effective medium approximation
() of our and the traditional random network model, and discuss
their comparative successes and shortcomings.Comment: Revised and reduced version with 17 LaTeX pages plus 8 JPEG figure
Statistics of level spacing of geometric resonances in random binary composites
We study the statistics of level spacing of geometric resonances in the
disordered binary networks. For a definite concentration within the
interval , numerical calculations indicate that the unfolded level
spacing distribution and level number variance have the
general features. It is also shown that the short-range fluctuation and
long-range spectral correlation lie between the profiles of the
Poisson ensemble and Gaussion orthogonal ensemble (GOE). At the percolation
threshold , crossover behavior of functions and is
obtained, giving the finite size scaling of mean level spacing and
mean level number , which obey the scaling laws, and .Comment: 11 pages, 7 figures,submitted to Phys. Rev.
Statistical and Dynamical Study of Disease Propagation in a Small World Network
We study numerically statistical properties and dynamical disease propagation
using a percolation model on a one dimensional small world network. The
parameters chosen correspond to a realistic network of school age children. We
found that percolation threshold decreases as a power law as the short cut
fluctuations increase. We found also the number of infected sites grows
exponentially with time and its rate depends logarithmically on the density of
susceptibles. This behavior provides an interesting way to estimate the
serology for a given population from the measurement of the disease growing
rate during an epidemic phase. We have also examined the case in which the
infection probability of nearest neighbors is different from that of short
cuts. We found a double diffusion behavior with a slower diffusion between the
characteristic times.Comment: 12 pages LaTex, 10 eps figures, Phys.Rev.E Vol. 64, 056115 (2001
Higher Order Effects in the Dielectric Constant of Percolative Metal-Insulator Systems above the Critical Point
The dielectric constant of a conductor-insulator mixture shows a pronounced
maximum above the critical volume concentration. Further experimental evidence
is presented as well as a theoretical consideration based on a phenomenological
equation. Explicit expressions are given for the position of the maximum in
terms of scaling parameters and the (complex) conductances of the conductor and
insulator. In order to fit some of the data, a volume fraction dependent
expression for the conductivity of the more highly conductive component is
introduced.Comment: 4 pages, Latex, 4 postscript (*.epsi) files submitted to Phys Rev.
Segregated tunneling-percolation model for transport nonuniversality
We propose a theory of the origin of transport nonuniversality in disordered
insulating-conducting compounds based on the interplay between microstructure
and tunneling processes between metallic grains dispersed in the insulating
host. We show that if the metallic phase is arranged in quasi-one dimensional
chains of conducting grains, then the distribution function of the chain
conductivities g has a power-law divergence for g -> 0 leading to nonuniversal
values of the transport critical exponent t. We evaluate the critical exponent
t by Monte Carlo calculations on a cubic lattice and show that our model can
describe universal as well nonuniversal behavior of transport depending on the
value of few microstructural parameters. Such segregated tunneling-percolation
model can describe the microstructure of a quite vast class of materials known
as thick-film resistors which display universal or nonuniversal values of t
depending on the composition.Comment: 8 pages, 5 figures (Phys. Rev. B - 1 August 2003)(fig1 replaced
Differential evolution for the offline and online optimization of fed-batch fermentation processes
The optimization of input variables (typically feeding trajectories over
time) in fed-batch fermentations has gained special attention, given the economic impact
and the complexity of the problem. Evolutionary Computation (EC) has been a
source of algorithms that have shown good performance in this task. In this chapter,
Differential Evolution (DE) is proposed to tackle this problem and quite promising
results are shown. DE is tested in several real world case studies and compared with
other EC algorihtms, such as Evolutionary Algorithms and Particle Swarms. Furthermore,
DE is also proposed as an alternative to perform online optimization, where the
input variables are adjusted while the real fermentation process is ongoing. In this case,
a changing landscape is optimized, therefore making the task of the algorithms more
difficult. However, that fact does not impair the performance of the DE and confirms
its good behaviour.(undefined
Genetic diversity and taxonomic aspects of wild carrot in France
If in France, genetic resources of cultivated carrot are well studied and preserved, the wild compartment of the species remains unknown and underutilized, while there are many populations of wild carrot on the French territory. Several collecting missions were undertaken from 2009 to 2013 to identify and collect populations, particularly in coastal areas, in continental France and Corsica. More than a hundred populations were collected, with a good distribution throughout the territory, and eleven taxa were identified. The status of these taxa is variable, with some very common and others underrepresented or specific to a given area. Morphological and molecular studies are developed in order to improve the knowledge of taxonomy and diversity. The study conducted with microsatellite markers showed a high genetic diversity at the intra-and inter-populations levels. The overall results show that the Mediterranean coast and Corsica exhibit a particularly high diversity. The work confirms the specific interest of some populations and the taxonomic separation into 2 subgroups carota and gummifer in Daucus carota L. This study will allow developing a strategy for management of genetic resources and their valorisation in breeding
On random symmetric matrices with a constraint: the spectral density of random impedance networks
We derive the mean eigenvalue density for symmetric Gaussian random N x N
matrices in the limit of large N, with a constraint implying that the row sum
of matrix elements should vanish. The result is shown to be equivalent to a
result found recently for the average density of resonances in random impedance
networks [Y.V. Fyodorov, J. Phys. A: Math. Gen. 32, 7429 (1999)]. In the case
of banded matrices, the analytical results are compared with those extracted
from the numerical solution of Kirchhoff equations for quasi one-dimensional
random impedance networks.Comment: 4 pages, 5 figure
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