## Publications

292 results found

DEGOND P, 1986, GLOBAL EXISTENCE OF SMOOTH SOLUTIONS FOR THE VLASOV-FOKKER-PLANCK EQUATION IN 1 SPACE AND 2 SPACE DIMENSIONS, *ANNALES SCIENTIFIQUES DE L ECOLE NORMALE SUPERIEURE*, Vol: 19, Pages: 519-542, ISSN: 0012-9593

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- Citations: 86

DEGOND P, 1985, LOCAL EXISTENCE OF SMOOTH SOLUTIONS OF THE VLASOV-MAXWELL EQUATIONS, AND APPROXIMATION BY THE SOLUTIONS OF THE VLASOV-POISSON EQUATIONS, *COMPTES RENDUS DE L ACADEMIE DES SCIENCES SERIE I-MATHEMATIQUE*, Vol: 301, Pages: 877-880, ISSN: 0764-4442

BARDOS C, NGOAN HT, DEGOND P, 1985, GLOBAL-SOLUTIONS FOR RELATIVISTIC VLASOV-POISSON EQUATIONS IN 3 SPACE VARIABLES, *COMPTES RENDUS DE L ACADEMIE DES SCIENCES SERIE I-MATHEMATIQUE*, Vol: 301, Pages: 265-268, ISSN: 0764-4442

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- Citations: 10

BARDOS C, DEGOND P, 1985, GLOBAL EXISTENCE FOR THE VLASOV-POISSON EQUATION IN 3 SPACE VARIABLES WITH SMALL INITIAL DATA, *ANNALES DE L INSTITUT HENRI POINCARE-ANALYSE NON LINEAIRE*, Vol: 2, Pages: 101-118, ISSN: 0294-1449

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- Citations: 158

DEGOND P, 1985, GLOBAL EXISTENCE OF SOLUTIONS OF THE VLASOV-FOKKER-PLANCK EQUATION IN 1-DIMENSION AND 2-DIMENSION, *COMPTES RENDUS DE L ACADEMIE DES SCIENCES SERIE I-MATHEMATIQUE*, Vol: 301, Pages: 73-76, ISSN: 0764-4442

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- Citations: 1

DEGOND P, 1983, APPEARANCE OF EIGEN MODES FOR THE LINEARIZED VLASOV-POISSON EQUATION, *COMPTES RENDUS DE L ACADEMIE DES SCIENCES SERIE I-MATHEMATIQUE*, Vol: 296, Pages: 969-972, ISSN: 0764-4442

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- Citations: 6

BARDOS C, DEGOND P, 1983, GLOBAL EXISTENCE AND ASYMPTOTIC-BEHAVIOR FOR THE SOLUTION OF THE VLASOV-POISSON EQUATION, *COMPTES RENDUS DE L ACADEMIE DES SCIENCES SERIE I-MATHEMATIQUE*, Vol: 297, Pages: 321-324, ISSN: 0764-4442

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- Citations: 6

Degond P, Ferreira MA, Merino-Aceituno S, et al., A new continuum theory for incompressible swelling materials

Swelling media (e.g. gels, tumors) are usually described by mechanicalconstitutive laws (e.g. Hooke or Darcy laws). However, constitutive relationsof real swelling media are not well known. Here, we take an opposite route andconsider a simple packing heuristics, i.e. the particles can't overlap. Wededuce a formula for the equilibrium density under a confining potential. Wethen consider its evolution when the average particle volume and confiningpotential depend on time under two additional heuristics: (i) any two particlescan't swap their position; (ii) motion should obey some energy minimizationprinciple. These heuristics determine the medium velocity consistently with thecontinuity equation. In the direction normal to the potential level sets thevelocity is related with that of the level sets while in the paralleldirection, it is determined by a Laplace-Beltrami operator on these sets. Thiscomplex geometrical feature cannot be recovered using a simple Darcy law.

Barker M, Degond P, Wolfram M-T, Comparing the best reply strategy and mean field games: the stationary case

Mean field games (MFGs) and the best reply strategy (BRS) are two methods ofdescribing competitive optimisation of systems of interacting agents. Thelatter can be interpreted as an approximation of the respective MFG system. Inthis paper we present a systematic analysis and comparison of the twoapproaches in the stationary case. We provide novel existence and uniquenessresults for the stationary boundary value problems related to the MFG and BRSformulations, and we present an analytical and numerical comparison of the twoparadigms in a variety of modelling situations.

Degond P, Asymptotic-Preserving Schemes for Fluid Models of Plasmas, *Panoramas et Syntheses 39-40, 2013, pp. 1-90*

These notes summarize a series of works related to the numericalapproximation of plasma fluid problems. We construct so-called'Asymptotic-Preserving' schemes which are valid for a large range of values(from very small to order unity) of the dimensionless parameters that appear inplasma fluid models. Specifically, we are interested in two parameters, thescaled Debye length which quantifies how close to quasi-neutrality the plasmais, and the scaled cyclotron period, which is inversely proportional to themagnetic field strength. We will largely focus on the ideas, in order to enablethe reader to apply these concepts to other situations.

Aoki K, Charrier P, Degond P, A hierarchy of models related to nanoflows and surface diffusion, *Kinetic and Related Models, 4 (2011), pp. 53-85*

In last years a great interest was brought to molecular transport problems atnanoscales, such as surface diffusion or molecular flows in nano orsub-nano-channels. In a series of papers V. D. Borman, S. Y. Krylov, A. V.Prosyanov and J. J. M. Beenakker proposed to use kinetic theory in order toanalyze the mechanisms that determine mobility of molecules in nanoscalechannels. This approach proved to be remarkably useful to give new insight onthese issues, such as density dependence of the diffusion coefficient. In thispaper we revisit these works to derive the kinetic and diffusion modelsintroduced by V. D. Borman, S. Y. Krylov, A. V. Prosyanov and J. J. M.Beenakker by using classical tools of kinetic theory such as scaling andsystematic asymptotic analysis. Some results are extended to less restrictivehypothesis.

Degond P, Dimarco G, Pareschi L, The Moment Guided Monte Carlo Method, *International Journal for Numerical Methods in Fluids, 67 (2011), pp. 189-213*

In this work we propose a new approach for the numerical simulation ofkinetic equations through Monte Carlo schemes. We introduce a new techniquewhich permits to reduce the variance of particle methods through a matchingwith a set of suitable macroscopic moment equations. In order to guarantee thatthe moment equations provide the correct solutions, they are coupled to thekinetic equation through a non equilibrium term. The basic idea, on which themethod relies, consists in guiding the particle positions and velocitiesthrough moment equations so that the concurrent solution of the moment andkinetic models furnishes the same macroscopic quantities.

Degond P, Hirstoaga S, Vignal M-H, The Vlasov model under large magnetic fields in the low-Mach number regime

This article is concerned with the kinetic modeling, by means of the Vlasovequation, of charged particles under the influence of a strong externalelectromagnetic field, i.e. when epsilon^2, the dimensionless cyclotron period,tends to zero. This leads us to split the velocity variable in the Vlasovequation into fluid and random components. The latter is supposed to have alarge magnitude of order 1/epsilon (which corresponds to the low Mach numberregime). In the limit epsilon -> 0, the resulting model is a hybrid model whichcouples a kinetic description of the microscopic random motion of the particlesto a fluid description of the macroscopic behavior of the plasma. Themicroscopic model is a first-order partial differential system for thedistribution function, which is averaged over the ultra-fast Larmor gyrationand the fast parallel motion along the magnetic field lines. The perpendicularcomponent (with respect to the magnetic field lines) of the bulk velocity isgoverned by the classical relations describing the E X B and diamagneticdrifts, while its parallel component satisfies an elliptic equation along themagnetic field lines.

Degond P, Mathematical models of collective dynamics and self-organization

In this paper, we begin by reviewing a certain number of mathematicalchallenges posed by the modelling of collective dynamics and self-organization.Then, we focus on two specific problems, first, the derivation of fluidequations from particle dynamics of collective motion and second, the study ofphase transitions and the stability of the associated equilibria.

Aceves-Sanchez P, Aymard B, Peurichard D, et al., A new model for the emergence of blood capillary networks

We propose a new model for the emergence of blood capillary networks. Weassimilate the tissue and extra cellular matrix as a porous medium, usingDarcy's law for describing both blood and intersticial fluid flows. Oxygenobeys a convection-diffusion-reaction equation describing advection by theblood, diffusion and consumption by the tissue. Discrete agents named capillaryelements and modelling groups of endothelial cells are created or deletedaccording to different rules involving the oxygen concentration gradient, theblood velocity, the sheer stress or the capillary element density. Oncecreated, a capillary element locally enhances the hydraulic conductivitymatrix, contributing to a local increase of the blood velocity and oxygen flow.No connectivity between the capillary elements is imposed. The coupling betweenblood, oxygen flow and capillary elements provides a positive feedbackmechanism which triggers the emergence of a network of channels of highhydraulic conductivity which we identify as new blood capillaries. We providetwo different, biologically relevant geometrical settings and numericallyanalyze the influence of each of the capillary creation mechanism in detail.All mechanisms seem to concur towards a harmonious network but the mostimportant ones are those involving oxygen gradient and sheer stress. A detaileddiscussion of this model with respect to the literature and its potentialfuture developments concludes the paper.

Degond P, Engel M, Liu J-G, et al., A Markov jump process modelling animal group size statistics

We translate a coagulation-framentation model, describing the dynamics ofanimal group size distributions, into a model for the population distributionand associate the \blue{nonlinear} evolution equation with a Markov jumpprocess of a type introduced in classic work of H.~McKean. In particular thisformalizes a model suggested by H.-S. Niwa [J.~Theo.~Biol.~224 (2003)] withsimple coagulation and fragmentation rates. Based on the jump process, wedevelop a numerical scheme that allows us to approximate the equilibrium forthe Niwa model, validated by comparison to analytical results by Degond et al.[J.~Nonlinear Sci.~27 (2017)], and study the population and size distributionsfor more complicated rates. Furthermore, the simulations are used to describestatistical properties of the underlying jump process. We additionally discussthe relation of the jump process to models expressed in stochastic differentialequations and demonstrate that such a connection is justified in the case ofnearest-neighbour interactions, as opposed to global interactions as in theNiwa model.

Degond P, Diez A, Frouvelle A, et al., Phase transitions and macroscopic limits in a BGK model of body-attitude coordination

In this article we investigate the phase transition phenomena that occur in amodel of self-organisation through body-attitude coordination. Here, thebody-attitude of an agent is modelled by a rotation matrix in $\mathbb{R}^3$ asin [Degond, Frouvelle, Merino-Aceituno, 2017]. The starting point of this studyis a BGK equation modelling the evolution of the distribution function of thesystem at a kinetic level. The main novelty of this work is to show that in thespatially homogeneous case, self-organisation may appear or not depending onthe local density of agents involved. We first exhibit a connection betweenbody-orientation models and models of nematic alignment of polymers in higherdimensional space from which we deduce the complete description of the possibleequilibria Then, thanks to a gradient-flow structure specific to this BGKmodel, we are able to prove the stability and the convergence towards theequilibria in the different regimes. We then derive the macroscopic modelsassociated to the stable equilibria in the spirit of [Degond, Frouvelle,Merino-Aceituno, 2017] and [Degond, Frouvelle, Liu, 2015].

BarrĂ© J, Degond P, Peurichard D, et al., Modelling pattern formation through differential repulsion

Motivated by experiments on cell segregation, we present a two-species modelof interacting particles, aiming at a quantitative description of thisphenomenon. Under precise scaling hypothesis, we derive from the microscopicmodel a macroscopic one and we analyze it. In particular, we determine therange of parameters for which segregation is expected. We compare ouranalytical results and numerical simulations of the macroscopic model to directsimulations of the particles, and comment on possible links with experiments.

Ferreira MA, Despin-Guitard E, Duarte F, et al., Interkinetic nuclear movements promote apical expansion in pseudostratified epithelia at the expense of apicobasal elongation, Publisher: Cold Spring Harbor Laboratory

<jats:title>Abstract</jats:title><jats:p>Pseudostratified epithelia (PSE) are a common type of columnar epithelia found in a wealth of embryonic and adult tissues such as ectodermal placodes, the trachea, the ureter, the gut and the neuroepithelium. PSE are characterized by the choreographed displacement of cells’ nuclei along the apicobasal axis according to phases of their cell cycle. Such movements, called interkinetic movements (INM) have been proposed to influence tissue expansion and shape and suggested as culprit in several congenital diseases such as CAKUT and esophageal atresia. INM rely on cytoskeleton dynamics just as adhesion, contractility and mitosis do. Therefore, longer term impairment of INM without affecting proliferation and adhesion is currently technically unachievable. Here we bypassed this hurdle by generating a 2D agent-based model of a proliferating PSE and compared its output to the growth of the chick neuroepithelium to assess the interplay between INM and these other important cell processes during growth of a PSE. We found that INM directly generates apical expansion and apical nuclear crowding. In addition, our data strongly suggest that apicobasal elongation of cells is not an emerging property of a proliferative PSE but rather requires a specific elongation program. We then discuss how such program might functionally link INM, tissue growth and differentiation.</jats:p><jats:sec><jats:title>Authors Summary</jats:title><jats:p>Pseudostratified epithelia (PSE) are a common type of epithelia characterized by the choreographed displacement of cells’ nuclei along the apicobasal axis during proliferation. These so-called interkinetic movements (INM) were proposed to influence tissue expansion and suggested as culprit in several congenital diseases. INM rely on cytoskeleton dynamics. Therefore, longer term impairment of INM without affecting proliferation and adhesion is currently t

Degond P, Merino-Aceituno S, Nematic alignment of self-propelled particles in the macroscopic regime

Starting from a particle model describing self-propelled particlesinteracting through nematic alignment, we derive a macroscopic model for theparticle density and mean direction of motion. We first propose a mean-fieldkinetic model of the particle dynamics. After diffusive rescaling of thekinetic equation, we formally show that the distribution function converges toan equilibrium distribution in particle direction, whose local density and meandirection satisfies a cross-diffusion system. We show that the system isconsistent with symmetries typical of a nematic material. The derivation iscarried over by means of a Hilbert expansion. It requires the inversion of thelinearized collision operator for which we show that the generalized collisioninvariants, a concept introduced to overcome the lack of momentum conservationof the system, plays a central role. This cross diffusion system poses many newchallenging questions.

Degond P, Jin S, Zhu Y, An Uncertainty Quantification Approach to the Study of Gene Expression Robustness

We study a chemical kinetic system with uncertainty modeling a generegulatory network in biology. Specifically, we consider a system of twoequations for the messenger RNA and micro RNA content of a cell. Our target isto provide a simple framework for noise buffering in gene expression throughmicro RNA production. Here the uncertainty, modeled by random variables, entersthe system through the initial data and the source term. We obtain a sharpdecay rate of the solution to the steady state, which reveals that the biologysystem is not sensitive to the initial perturbation around the steady state.The sharp regularity estimate leads to the stability of the generalizedPolynomial Chaos stochastic Galerkin (gPC-SG) method. Based on the smoothnessof the solution in the random space and the stability of the numerical method,we conclude the gPC-SG method has spectral accuracy. Numerical experiments areconducted to verify the theoretical findings.

Degond P, Henkes S, Yu H, Self-Organized Hydrodynamics with nonconstant velocity

Motivated by recent experimental and computational results that show amotility-induced clustering transition in self-propelled particle systems, westudy an individual model and its corresponding Self-Organized Hydrodynamicmodel for collective behaviour that incorporates a density-dependent velocity,as well as inter-particle alignment. The modal analysis of the hydrodynamicmodel elucidates the relationship between the stability of the equilibria andthe changing velocity, and the formation of clusters. We find, in agreementwith earlier results for non-aligning particles, that the key criterion forstability is $(\rho v(\rho))'> 0$, i.e. a non-rapid decrease of velocity withdensity. Numerical simulation for both the individual and hydrodynamic modelswith a velocity function inspired by experiment demonstrates the validity ofthe theoretical results.

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