634 research outputs found
Microscopic models of traveling wave equations
Reaction-diffusion problems are often described at a macroscopic scale by
partial derivative equations of the type of the Fisher or
Kolmogorov-Petrovsky-Piscounov equation. These equations have a continuous
family of front solutions, each of them corresponding to a different velocity
of the front. By simulating systems of size up to N=10^(16) particles at the
microscopic scale, where particles react and diffuse according to some
stochastic rules, we show that a single velocity is selected for the front.
This velocity converges logarithmically to the solution of the F-KPP equation
with minimal velocity when the number N of particles increases. A simple
calculation of the effect introduced by the cutoff due to the microscopic scale
allows one to understand the origin of the logarithmic correction.Comment: 11 pages, 3 figure
Ground state energy of a non-integer number of particles with delta attractive interactions
We show how to define and calculate the ground state energy of a system of
quantum particles with delta attractive interactions when the number of
particles n$is non-integer. The question is relevant to obtain the probability
distribution of the free energy of a directed polymer in a random medium. When
one expands the ground state energy in powers of the interaction, all the
coefficients of the perturbation series are polynomials in n, allowing to
define the perturbation theory for non-integer n. We develop a procedure to
calculate all the cumulants of the free energy of the directed polymer and we
give explicit, although complicated, expressions of the first three cumulants.Comment: 11 pages, no figur
Global existence for a free boundary problem of Fisher-KPP type
Motivated by the study of branching particle systems with selection, we
establish global existence for the solution of the free boundary
problem when the initial condition is
non-increasing with as and as . We construct the solution as the limit of a sequence , where each is the solution of a Fisher-KPP equation with same
initial condition, but with a different non-linear term.
Recent results of De Masi \textit{et al.}~\cite{DeMasi2017a} show that this
global solution can be identified with the hydrodynamic limit of the so-called
-BBM, {\it i.e.} a branching Brownian motion in which the population size is
kept constant equal to by killing the leftmost particle at each branching
event
Traveling discontinuity at the quantum butterfly front
We formulate a kinetic theory of quantum information scrambling in the
context of a paradigmatic model of interacting electrons in the vicinity of a
superconducting phase transition. We carefully derive a set of coupled partial
differential equations that effectively govern the dynamics of information
spreading in generic dimensions. They exhibit traveling wave solutions that are
discontinuous at the boundary of the light cone, and have a perfectly causal
structure where the solutions do not spill outside of the light cone.Comment: 31 pages, 7 figure
Le management en situation de complexité et d’incertitude. Apport de la Recherche et Développement
La seconde révolution industrielle a vu se développer des méthodes de management adaptées aux contextes successivement traversés, d’abord la nécessité de produire en masse, puis d’améliorer la qualité et la satisfaction des clients, puis d’améliorer la performance opérationnelle en réponse à la globalisation des marchés. Initialement inspirées d’une pensée mécaniste rationnelle, ces approches se sont progressivement étoffées en y intégrant d’autres dimensions, la psychologie, la sociologie, l’analyse systémique. Dans les années 1990, les entreprises s’étaient profondément remises en cause, avaient mis en place des modes de fonctionnement raffinés, mais deux nouveaux paramètres ont bouleversé leur environnement : la complexité et l’incertitude. Les entreprises de la troisième révolution industrielle ont d’emblée intégré ces paramètres, et mis en place de nouveaux modes de management. Pour autant, ceux-ci peuvent ne pas répondre à des activités où l’erreur peut être mortelle, où le bug n’est pas acceptable. Entre souci de performance et de bien-être au travail, entre principe de précaution et principe d’expérimentation, la troisième révolution industrielle cherche encore ses approches, alors qu’une quatrième révolution est peut-être déjà là et annonce de nouveaux bouleversements. Dans cette perspective marquée par une complexité et une incertitude croissantes, la Recherche et Développement présente un intérêt particulier puisque sa vocation, précisément, consiste à affronter le complexe et l’incertain. Dans cet article, nous dégagerons les grands fondamentaux du processus R&D, et analyserons en quoi ils peuvent servir de benchmark au management contemporain, en lui apportant des sources d’inspiration
A low Complexity Wireless Gigabit Ethernet IFoF 60 GHz H/W Platform and Issues
6 pagesInternational audienceThis paper proposes a complete IFoF system architecture derived from simplified IEEE802.15.3c PHY layer proposal to successfully ensure near 1 Gbps on the air interface. The system architecture utilizes low complexity baseband processing modules. The byte/frame synchronization technique is designed to provide a high value of preamble detection probability and a very small value of the false detection probability. Conventional Reed-Solomon RS (255, 239) coding is used for Channel Forward Error Correction (FEC). Good communication link quality and Bit Error Rate (BER) results at 875 Mbps are achieved with directional antennas
An unusual occurrence of Nautilus macromphalus in a cenote in the Loyalty Islands (New Caledonia)
Exploration of a landlocked cenote on Lifou (Loyalty Islands) revealed 37 shells of the cephalopod Nautilus macromphalus Sowerby, 1849, in saltwater on the cenote floor, approximately 40 m below the water surface. The occurrence of these shells is unusual because N. macromphalus is restricted to the open marine waters surrounding the island. All of the shells are mature, and nearly all of them are unbroken, with faded red-brown color stripes. We analyzed seven shells to determine their age. Radiocarbon dating yielded ages of 6380¡30 to 7095¡30 y BP. The 238U-series radionuclides 210Pb (half-life 522.3 y) and 226Ra (half-life 51600 y) also were measured. Two of the samples showed radioactive equilibrium between the nuclides, consistent with the old radiocarbon dates, but the other five samples showed excess 210Pb. When corrected for radioactive decay, the 226Ra activities were much greater than those found in living Nautilus. We conclude that exposure to high activities of 222Rn and 226Ra in the salty groundwater of the cenote altered the activities originally incorporated into the shells. Human placement of the shells in the cavity is rejected based on their radiocarbon age and the geometry of the cenote. The most probable explanation is that the animals entered the flooded karstic system through a connection on the seaward side at approximately 7,000 y BP, during an interval of slowly rising sea level. Unable to find an exit and/or due to anoxic bottom waters, the animals were trapped and died inside. The open connection with the sea persisted for ,700 y, but after ,6400 y BP, the connection was lost, probably due to a roof collapse. This is a rare example of Nautilus in a karstic coastal basin and provides a minimum age for the appearance of N. macromphalus in the Loyalty Islands
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