Sharp Interface Limits for Non-Local Anisotropic Interactions

We prove a convexity property of the surface tension corresponding to a non-local, anisotropic free-energy functional of van der Waals type which implies that the Wulff shape is strictly convex and smooth. We also prove that the transport coefficients of the limiting anisotropic motion by mean curvature obtained in [33] are strictly positive and equal to the stiffness parameters determined by the surface tension.     

Sharp Observability Estimates for Heat Equations

The goal of this article is to derive new estimates for the cost of observability of heat equations. We have developed a new method allowing one to show that when the corresponding wave equation is observable, the heat equation is also observable. This method allows one to describe the explicit dependence of the observability constant on the geometry of the problem (the domain in which the heat process evolves and the observation subdomain). We show that our estimate is sharp in some cases, particularly in one space dimension and in the multi-dimensional radially symmetric case. Our result extends those in Fattorini and Russell (Arch Rational Mech Anal 43:272–292, 1971) to the multi-dimensional setting and improves those available in the literature, namely those by Miller (J Differ Equ 204(1):202–226, 2004; SIAM J Control Optim 45(2):762–772, 2006; Atti Accad Naz Lincei Cl Sci Fis Mat Natur Rend Lincei (9) Mat Appl 17(4):351–366, 2006) and Tenenbaum and Tucsnak (J Differ Equ 243(1):70–100, 2007). Our approach is based on an explicit representation formula of some solutions of the wave equation in terms of those of the heat equation, in contrast to the standard application of transmutation methods, which uses a reverse representation of the heat solution in terms of the wave one. We shall also explain how our approach applies and yields some new estimates on the cost of observability in the particular case of the unit square observed from one side. We will also comment on the applications of our techniques to controllability properties of heat-type equations.     

On Aspects of Asymptotics for Plate Equations

In this paper some initial-boundary value problems for plate equations will be studied. These initial-boundary value problems can be regarded as simple models describing free oscillations of plates on elastic foundations or of plates to which elastic springs are attached on the boundary. It is assumed that the foundations and springs have a different behavior for compression and for extension. An approximation for the solution of the initial-boundary value problem will be constructed by using a two-timescales perturbation method. For specific parameter values it turns out that complicated internal resonances occur.     

Anisotropic rectangular nonconforming finite element analysis for Sobolev equations

An anisotropic rectangular nonconforming finite element method for solving the Sobolev equations is discussed under semi-discrete and full discrete schemes. The corresponding optimal convergence error estimates and superclose property are derived, which are the same as the traditional conforming finite elements. Furthermore, the global superconvergence is obtained using a post-processing technique. The numerical results show the validity of the theoretical analysis.     

Sharp Asymptotics for KPP Pulsating Front Speed-Up and Diffusion Enhancement by Flows

We study KPP pulsating front speed-up and effective diffusivity enhancement by general periodic incompressible flows. We prove the existence of and determine the limits c^*_e(A)/A{c^*_{e}(A)/A} and D _e (A)/A ² as the flow amplitude A → ∞, with c^*_e(A){c^*_{e}(A)} the minimal front speed and D _e (A) the effective diffusivity in direction e.     

On Solutions with Power Asymptotics for Differential Equations with Exponential Nonlinearity

We establish necessary and sufficient conditions for the existence of solutions with power asymptotics for two-term differential equations with exponential nonlinearity.     

On a Sharp Sobolev‐Type Inequality on Two-Dimensional Compact Manifolds

Motivated by the asymptotic analysis of double vortex condensates in the Chern‐Simons‐Higgs theory, we construct a suitable minimizing sequence for a sharp Sobolev inequality “à la Moser” for two‐dimensional compact manifolds. As a consequence, we first obtain a direct proof of the sharp character of such an inequality. Secondly, and more interestingly, we use such minimizing sequence to show that for the flat torus the corresponding extremal problem attains its infimum. (Accepted April 6, 1998)     

Equations of an Elastic Anisotropic Layer

Differential equations of an elastic orthotropic layer are constructed on the basis of expansion of the solutions of the elasticity theory in terms of the Legendre polynomials. The order of the system of differential equations is independent of the form of the boundary conditions on the layer surfaces, which allows a correct formulation of conditions on contact surfaces.     

Global Regularity for the Two-Dimensional Anisotropic Boussinesq Equations with Vertical Dissipation

This paper establishes the global in time existence of classical solutions to the two-dimensional anisotropic Boussinesq equations with vertical dissipation. When only vertical dissipation is present, there is no direct control on the horizontal derivatives and the global regularity problem is very challenging. To solve this problem, we bound the derivatives in terms of the ${L^\infty}$ -norm of the vertical velocity v and prove that ${\|v\|_{L^{r}}}$ with ${2\leqq r < \infty}$ does not grow faster than ${\sqrt{r \log r}}$ at any time as r increases. A delicate interpolation inequality connecting ${\|v\|_{L^\infty}}$ and ${\|v\|_{L^r}}$ then yields the desired global regularity.     

On Asymptotics of Solutions of Semiexplicit Systems of Differential Equations

We study the problem of the existence of analytic solutions of a certain semiexplicit system of differential equations and obtain sufficient conditions for the existence of analytic solutions of the Cauchy problem in the neighborhood of a singular point.__________Translated from Neliniini Kolyvannya, Vol. 8, No. 1, pp. 132–144, January–March, 2005.     

The Anisotropic Lagrangian Averaged Euler and Navier-Stokes Equations

 The purpose of this paper is twofold. First, we give a derivation of the Lagrangian averaged Euler (LAE-α) and Navier-Stokes (LANS-α) equations. This theory involves a spatial scale α and the equations are designed to accurately capture the dynamics of the Euler and Navier-Stokes equations at length scales larger than α, while averaging the motion at scales smaller than α. The derivation involves an averaging procedure that combines ideas from both the material (Lagrangian) and spatial (Eulerian) viewpoints. This framework allows the use of a variant of G. I. Taylor's ``frozen turbulence' hypothesis as the foundation for the model equations; more precisely, the derivation is based on the strong physical assumption that fluctutations are frozen into the mean flow. In this article, we use this hypothesis to derive the averaged Lagrangian for the theory, and all the terms up to and including order α² are accounted for. The equations come in both an isotropic and anisotropic version. The anisotropic equations are a coupled system of PDEs (partial differential equations) for the mean velocity field and the Lagrangian covariance tensor. In earlier works by Foias, Holm & Titi [10], and ourselves [16], an analysis of the isotropic equations has been given. In the second part of this paper, we establish local in time well-posedness of the anisotropic LANS-α equations using quasilinear PDE type methods. (Accepted September 2, 2002) Published online November 26, 2002 Dedicated to Stuart Antman on the occasion of his 60th birthday Communicated by S. MüLLER     

Asymptotics of the Critical Regimes of Internal Gravity Wave Generation

The problem of constructing an asymptotic representation of the solution of the internal gravity wave field excited by a source moving at a velocity close to the maximum group velocity of the individual wave mode is considered. For the critical regimes of individual mode generation the asymptotic representation of the solution obtained is expressed in terms of a zero-order Macdonald function. The results of numerical calculations based on the exact and asymptotic formulas are given.     

Sobolev Periodic Solutions of Nonlinear Wave Equations in Higher Spatial Dimensions

We prove the existence of Cantor families of periodic solutions for nonlinear wave equations in higher spatial dimensions with periodic boundary conditions. We study both forced and autonomous PDEs. In the latter case our theorems generalize previous results of Bourgain to more general nonlinearities of class C ^k and assuming weaker non-resonance conditions. Our solutions have Sobolev regularity both in time and space. The proofs are based on a differentiable Nash–Moser iteration scheme, where it is sufficient to get estimates of interpolation-type for the inverse linearized operators. Our approach works also in presence of very large “clusters of small divisors”.     

Sharp Estimates of Bounded Solutions to Some Second-order Forced Dissipative Equations

Résumé A l’aide d’inégalités différentielles, on établit une estimation proche de l’optimalité pour la norme dans de l’unique solution bornée de u′′ + cu′ + Au = f(t) lorsque A = A ^* ≥ λ I est un opérateur borné ou non sur un espace de Hilbert réel H, V = D(A ^1/2) et λ, c sont des constantes positives, tandis que . By using differential inequalities, a close-to-optimal bound of the unique bounded solution of u′′ + cu′ + Au = f(t) is obtained whenever A = A ^* ≥ λ I is a bounded or unbounded linear operator on a real Hilbert space H, V = D(A ^1/2) and λ, c are positive constants, while .

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简化Navier—Stokes方程的IFT理论的各向异性张力有限元法

本文首先讨论简化Navier-Stokes IFT方程组的有限元离散方式,然后对其广义解进行分析,并从而利用与之相匹配的各向异性张力单元对流函数—涡量方程进行计算。通过平板层流和台阶绕流两个算例的分析,证明这种与IFT理论相匹配的有限单元算法是成功的。     

On Asymptotics of Solutions of Nonlinear Differential Equations of the Second Order

We study the problem of asymptotics of unbounded solutions of differential equations of the form y″ = α₀ p(t)ϕ(y), where α₀ ∈ {−1, 1}, p: [a, ω[→]0, +∞[, −∞ < a < ω ≤ +∞, is a continuous function, and ϕ: [y ₀, +∞[→]0, +∞[ is a twice continuously differentiable function close to a power function in a certain sense.__________Translated from Neliniini Kolyvannya, Vol. 8, No. 1, pp. 18–28, January–March, 2005.     

Second-Order Fredholm Equations for the First Boundary-Value Problem in the Two-Dimensional Anisotropic Theory of Elasticity

A complete potential theory is constructed for the first boundary-value problem in the two-dimensional anisotropic theory of elasticity (the force vector is specified on the boundary) in a bounded domain on a plane with a Lyapunov boundary. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 2, pp. 85–94, March–April, 2006.     

Invariant Manifolds and the Long-Time Asymptotics of the Navier-Stokes and Vorticity Equations on R2

We construct finite-dimensional invariant manifolds in the phase space of the Navier-Stokes equation on R ² and show that these manifolds control the long-time behavior of the solutions. This gives geometric insight into the existing results on the asymptotics of such solutions and also allows us to extend those results in a number of ways.     

On the Euler Equations in the Critical Triebel-Lizorkin Spaces

In this paper we study the initial value problem of the incompressible Euler equations in ⁿ for initial data belonging to the critical Triebel-Lizorkin spaces, i.e., v ₀ F ⁿ⁺¹ _1,q , q[1, ]. We prove the blow-up criterion of solutions in F ⁿ⁺¹ _1,q for n=2,3. For n=2, in particular, we prove global well-posedness of the Euler equations in F ³ _1,q , q[1, ]. For the proof of these results we establish a sharp Moser-type inequality as well as a commutator-type estimate in these spaces. The key methods are the Littlewood-Paley decomposition and the paradifferential calculus by J. M. Bony.