117 research outputs found
Pointwise regularity of the free boundary for the parabolic obstacle problem
We study the parabolic obstacle problem \lap u-u_t=f\chi_{\{u>0\}}, \quad
u\geq 0,\quad f\in L^p \quad \mbox{with}\quad f(0)=1 and obtain two
monotonicity formulae, one that applies for general free boundary points and
one for singular free boundary points. These are used to prove a second order
Taylor expansion at singular points (under a pointwise Dini condition), with an
estimate of the error (under a pointwise double Dini condition). Moreover,
under the assumption that is Dini continuous, we prove that the set of
regular points is locally a (parabolic) -surface and that the set of
singular points is locally contained in a union of (parabolic) manifolds
Homogenization of the Peierls-Nabarro model for dislocation dynamics
This paper is concerned with a result of homogenization of an
integro-differential equation describing dislocation dynamics. Our model
involves both an anisotropic L\'{e}vy operator of order 1 and a potential
depending periodically on u/\ep. The limit equation is a non-local
Hamilton-Jacobi equation, which is an effective plastic law for densities of
dislocations moving in a single slip plane.Comment: 39 page
The method of differential contractions
In this Note we present a general and fairly simple method to design families of contractions for nonlinear partial differential equations, either of evolution type, or of stationary type. As a particular example, we apply this method to the porous medium equation, for which we get new contractions. This method opens new directions to explore
A kinetic formulation of moving fronts and application to dislocations dynamics
In this article, we consider hypersurfaces moving with normal velocity depending on the time-space coordinates and on the normal to the hypersurface. We naturally define a measure associated to this hypersurface. This measure is defined on a suitable space/unit normal/curvature configuration space. We show that, while the hypersurface stays smooth, then the measure is a solution to a linear transport equation, that we call a kinetic formulation. In the particular case of curves moving in the plane, we get a simple kinetic formulation. With this kinetic formulation in hands, it is then easy to complete the models of dislocations densities that were proposed in the 60's. As a consequence, we therefore propose a closed mean field model for the dynamics of dislocations densities
A junction condition by specified homogenization and application to traffic lights
Given a coercive Hamiltonian which is quasi-convex with respect to the
gradient variable and periodic with respect to time and space at least "far
away from the origin", we consider the solution of the Cauchy problem of the
corresponding Hamilton-Jacobi equation posed on the real line. Compact
perturbations of coercive periodic quasi-convex Hamiltonians enter into this
framework for example. We prove that the rescaled solution converges towards
the solution of the expected effective Hamilton-Jacobi equation, but whose
"flux" at the origin is "limited" in a sense made precise by the authors in
\cite{im}. In other words, the homogenization of such a Hamilton-Jacobi
equation yields to supplement the expected homogenized Hamilton-Jacobi equation
with a junction condition at the single discontinuous point of the effective
Hamiltonian. We also illustrate possible applications of such a result by
deriving, for a traffic flow problem, the effective flux limiter generated by
the presence of a finite number of traffic lights on an ideal road. We also
provide meaningful qualitative properties of the effective limiter.Comment: 41 page
Uniqueness and existence of spirals moving by forced mean curvature motion
In this paper, we study the motion of spirals by mean curvature type motion
in the (two dimensional) plane. Our motivation comes from dislocation dynamics;
in this context, spirals appear when a screw dislocation line reaches the
surface of a crystal. The first main result of this paper is a comparison
principle for the corresponding parabolic quasi-linear equation. As far as
motion of spirals are concerned, the novelty and originality of our setting and
results come from the fact that, first, the singularity generated by the
attached end point of spirals is taken into account for the first time, and
second, spirals are studied in the whole space. Our second main result states
that the Cauchy problem is well-posed in the class of sub-linear weak
(viscosity) solutions. We also explain how to get the existence of smooth
solutions when initial data satisfy an additional compatibility condition.Comment: This new version contains new results: we prove that the weak
(viscosity) solutions of the Cauchy problem are in fact smooth. This is a
consequence of some gradient estimates in time and spac
Explicit phase diagram for a one-dimensional blister model
In this article, we consider a simple one-dimensional variational model,
describing the delamination of thin films under cooling. We characterize the
global minimizers, which correspond to films of three possible types: non
delaminated, partially delaminated (called blisters), or fully delaminated. Two
parameters play an important role: the length of the film and the cooling
parameter. In the phase plane of those two parameters, we classify all the
minimizers. As a consequence of our analysis, we identify explicitly the
smallest possible blisters for this model
The Peierls-Nabarro model as a limit of a Frenkel-Kontorova model
We study a generalization of the fully overdamped Frenkel-Kontorova model in
dimension This model describes the evolution of the position of each
atom in a crystal, and is mathematically given by an infinite system of coupled
first order ODEs. We prove that for a suitable rescaling of this model, the
solution converges to the solution of a Peierls-Nabarro model, which is a
coupled system of two PDEs (typically an elliptic PDE in a domain with an
evolution PDE on the boundary of the domain). This passage from the discrete
model to a continuous model is done in the framework of viscosity solutions
A new contraction family for porous medium and fast diffusion equation
In this paper, we present a surprising two-dimensional contraction family for
porous medium and fast diffusion equations. This approach provides new a priori
estimates on the solutions, even for the standard heat equation
Derivation and study of dynamical models of dislocation densities
In this paper, starting from the microscopic dynamics of isolated
dislocations, we explain how to derive formally mean field models for the
dynamics of dislocation densities. Essentially these models are tranport
equations, coupled with the equations of elasticity. Rigorous results of
existence of solutions are presented for some of these models and the main
ideas of the proofs are given
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