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Vol 12 - Numéro spécial : AUS-ICMS 2020

Avancées en Mathématiques Pures et Appliquées

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This special volume of Advances in Pure and Applied Mathematics “APAM” is dedicated to the third International Conference on Mathematics and Statistics (AUS-ICMS2020) held at the American University of Sharjah in the United Arab Emirates, 6th -9th February 2020. The volume consists of eight invited peer reviewed articles covering different areas of mathematics by accomplished mathematicians.

Matrices de Permutation et au-delà

This is an exposition of my keynote talk given at The Third International Conference on Mathematics and Statistics : AUS-ICMS February 2020, Sharjah, UAE. It concerns permutations and permutation matrices, and some of their generalizations. It is not intended to be comprehensive in the topics discussed, but to highlight some aspects. The final section discusses a number of open problems and conjectures. A number of references are provided to get one started on a more comprehensive study.

Sur l’excès de la moyenne quadratique des erreurs associées à des fenêtres adaptatives dans l’estimation non-paramétrique de la tendance

We consider the problem of the optimal selection of the smoothing parameter $$${h}$$$ in kernel estimation of a trend in nonparametric regression models with strictly stationary errors. We suppose that the errors are stochastic volatility sequences. Three types of volatility sequences are studied : the log-normal volatility, the Gamma volatility and the log-linear volatility with Bernoulli innovations. We take the weighted average squared error (ASE) as the global measure of performance of the trend estimation using $$${h}$$$ and we study two classical criteria for selecting $$${h}$$$ from the data, namely the adjusted generalized cross validation and Mallows-type criteria. We establish the asymptotic distribution of the gap between the ASE evaluated at one of these selectors and the smallest possible ASE. A Monte-Carlo simulation for a log-normal stochastic volatility model illustrates that this asymptotic approximation can be accurate even for small sample sizes.

L’équation de Laplace-Beltrami sur une hypersurface avec un bord de Lipschitz

Main objective of the present paper is to prove solvability of the Dirichlet, Neumann and Mixed boundary value problems for an anisotropic Laplace-Beltrami equation on a hypersurface $$${C}$$$ with the Lipschitz boundary $$${\Gamma=∂C}$$$ in the classical $$${𝕎^1(C)}$$$ space setting.

Estimation de l’explosion et de temps d’existence pour l’équation de Tricomi généralisée avec des non-linéarités mixtes

We study in this article the blow-up of the solution of the generalized Tricomi equation in the presence of two mixed nonlinearities, namely we consider
$$${(Tr) \hspace{1cm} u_{tt}-t^{2m}\Delta u=}$$$|$$${u_t}$$$|$$${^p+}$$$|$$${u}$$$|$$${^q}$$$, $$${\quad \mbox{in}\ \mathbb{R}^N\times[0,\infty)}$$$,
with small initial data, where $$${m ≥ 0}$$$.
For the problem $$${(Tr)}$$$ with $$${m = 0}$$$, which corresponds to the uniform wave speed of propagation, it is known that the presence of mixed nonlinearities generates a new blow-up region in comparison with the case of a one nonlinearity (|$$${u_t}$$$|$$${^p}$$$ or |$$${u}$$$|$$${^q}$$$). We show in the present work that the competition between the two nonlinearities still yields a new blow region for the Tricomi equation $$${(Tr)}$$$ with $$${m ≥ 0}$$$, and we derive an estimate of the lifespan in terms of the Tricomi parameter $$${m}$$$. As an application of the method developed for the study of the equation $$${(Tr)}$$$ we obtain with a different approach the same blow-up result as in [18] when we consider only one time-derivative nonlinearity, namely we keep only |$$${u_t}$$$|$$${^p}$$$ in the right-hand side of $$${(Tr)}$$$.

Un modèle de population multirégional et non linéaire structuré par la taille avec coagulation et effets verticaux

A general nonlinear model describing the evolution of size-structured populations influenced by coagulation and vertical effects is presented. The nonlinear population dynamics occur in a multi-region setting in which the transfer, coagulation, and vital rates of an individual depend on a vector describing conditions in the environment. The model for these dynamics consists of a system of two-dimensional nonlinear-nonlocal hyperbolic partial differential equations coupled with a system of one-dimensional nonlocal differential equations parametrized by a vertical location coordinate $$${z}$$$ describing the environmental time-dynamics. A finite difference approximation approach is employed to study the wellposedness of the model and convergence of the scheme to the unique weak solution is established. Several examples are presented to illustrate the generality of the model and to motivate applications.

L’analyse d’une méthode semi-lagrangienne et éléments finis pour les problémes de convection-diffusion en milieux poreux

L’objectif de ce travail est l’étude d’une méthode de résolution numérique par éléments finis semilagrangiennes afin de résoudre les problémes évolutifs de convection-diffusion issus des milieux poreux. La méthode proposée permet d’utiliser une approximation par éléments finis d’ordre égal pour toutes les solutions du probléme. En outre, la condition standard de Courant-Friedrichs-Lewy est assouplie avec le traitement lagrangien des termes de convection, et les erreurs de troncature sont réduites dans la partie diffusion-réaction du probléme. Dans cette étude, une analyse de la convergence et de la stabilité de la méthode proposée est aussi présentée, ainsi que les estimations des erreurs dans la norme $$${L}$$$2 dérivées pour toutes les solutions numériques. Les tests numériques sont illustrées par quelques exemples afin de vérifier les estimations théoriques et de démontrer la grande précision et l’efficacité de la méthode proposée.

Principes du maximum et problèmes surdéterminés des équations Hessiennes

In this article we investigate some Hessian type equations. Our main aim is to derive new maximum principles for some suitable P-functions, in the sense of L.E. Payne, that is for some appropriate functional combinations of $$${u(x)}$$$ and its derivatives, where $$${u(x)}$$$ is a solution of the given Hessian type equations. To find the most suitable P-functions, we first investigate the special case of a ball, where the solution of our Hessian equations is radial, since this case gives good hints on the best functional to be considered later, for general domains. Next, we construct some elliptic inequalities for the well-chosen P-functions and make use of the classical maximum principles to get our new maximum principles. Finally, we consider some overdetermined problems and show that they have solutions when the underlying domain has a certain shape (spherical or ellipsoidal).

Sur les équations de réplicateur avec fonctions à gain non-linéaire définies par les modèles de Rick

An evolutionary game is usually identified by a smooth (possibly nonlinear) payoff function. In this paper, we propose a model of evolutionary game in which the nonlinear payoff functions are defined by the Ricker models. Namely, in the proposed model, the biological fitness of the pure strategy will increase according to the Ricker model. Motivated by some models of economics of transportation and communication networks, in order to observe the evolutionary bifurcation diagram, we also control the nonlinear payoff functions in two different regimes : positive and negative. One of the interesting feature of the model is that if we switch the controlling parameter from positive to negative regime then the set of local evolutionarily stable strategies (ESSs) changes from one set to another one. We also study the dynamics and stability analysis of the discrete-time replicator equation governed by the proposed nonlinear payoff function. In the long-run time, the following scenario can be observed : (i) in the positive regime, the active dominating pure strategies will outcompete other strategies and only they will survive forever ; (ii) in the negative regime, all active pure strategies will coexist together and they will survive forever.