We present in this paper a stability study concerning finite volume schemesapplied to the two-dimensional Maxwell system, using rectangular or triangular meshes. A stability condition is proved for thefirst-order upwind scheme on a rectangular mesh. Stability comparisonsbetween the Yee scheme and the finite volume formulation are proposed.We also compare the stability domains obtained when considering theMaxwell system and the convection equation.

We propose and analyse a abstract framework for augmented mixed formulations.We give a priori error estimate in the general case: conforming and nonconforming approximations with or without numerical integration.Finally, a posteriori error estimator is given. An example of stabilized formulation for Stokes problem is analysed.

The idea of replacing a divergence constraint by a divergence boundarycondition is investigated. The connections between the formulations areconsidered in detail. It is shown that the most common methods of usingdivergence boundary conditions do not always work properly. Necessaryand sufficient conditions for the equivalence of the formulations aregiven.

In this paper, a class of cell centered finite volume schemes, on general unstructured meshes, for a linear convection-diffusion problem, is studied. The convection and the diffusion are respectively approximated by means of an upwind scheme and the so called diamond cell method [4]. Our main result is an error estimate oforder h, assuming only the W2,p (for p>2) regularity of thecontinuous solution, on a mesh of quadrangles. The proof is based on anextension of the ideas developed in [12]. Some newdifficulties arise here, due to the weak regularity of the solution, and thenecessity to approximate the entire gradient, and not only its normalcomponent, as in [12].

We consider the evolution of a set $\Lambda\subset \mathbb R^2$ according to theHuygens principle: i.e. the domain at time t>0, Λt, is the set ofthe points whose distance from Λ is lower thant. We give some general results for this evolution,with particular care given to the behavior of the perimeter of theevoluted set as a function of time. We define a class of sets (non-trapping sets) for which the perimeter is a continuous function of t, andwe give an algorithm to approximate the evolution.Finally we restrict our attention to the class of sets for which the turning angle of the boundary is greater than -π (see [2]).For this class of sets we prove that the perimeter is a Lipschitz-continuous function of t. This evolution problem is relevant for the applicationsbecause it is used as a model for solid fuel combustion.

Error estimates in L∞(0,T;L 2(Ω)), L∞(0,T;L 2(Ω)2),L∞(0,T;L ∞(Ω)), L∞(0,T;L∞ (Ω)2), Ω in ${\mathbb R}^2$ , are derived for a mixed finite element method for the initial-boundary value problem for integro-differential equation $$u_t={\rm div}\{a\bigtriangledown u+\int^t_0b_1\bigtriangledown u{\rm d}\tau+\int^t_0{\bf c}u{\rm d}\tau\}+f$$ based on the Raviart-Thomas space V h x Wh ⊂ H(div;Ω) x L2(Ω). Optimal order estimates are obtained for theapproximation of u,ut in L∞(0,T;L 2(Ω)) and the associated velocity p in L∞(0,T;L 2(Ω)2), divp in L∞(0,T;L 2(Ω)). Quasi-optimal order estimates are obtained for the approximation of u in L∞ (0,T;L ∞(Ω)) and p in L∞(0,T;L ∞(Ω)2.

We present a family of high-order, essentially non-oscillatory, central schemes for approximating solutions of hyperbolic systems of conservation laws. These schemes are based on a new centered version of the Weighed Essentially Non-Oscillatory (WENO) reconstruction of point-valuesfrom cell-averages, which is then followed by an accurate approximationof the fluxes via a natural continuous extension of Runge-Kutta solvers.We explicitly construct the third and fourth-order scheme and demonstrate theirhigh-resolution properties in several numerical tests.

The equilibrium configurations of a one-dimensional variational model thatcombines terms expressing the bulk energy of a deformable crystal and itssurface energy are studied. After elimination of the displacement, theproblem reduces to the minimization of a nonconvex and nonlocal functional ofa single function, the thickness. Depending on a parameter which strengthensone of the terms comprising the energy at the expense of the other, it isshown that this functional may have a stable absolute minimum or only aminimizing sequence in which the term corresponding to the bulk energy isforced to zero by the production of a crack in the material.

In this paper we are interested in the numerical modelingof absorbing ferromagnetic materialsobeying the non-linear Landau-Lifchitz-Gilbert law with respect to the propagation and scattering of electromagnetic waves.In this workwe consider the 1D problem. We first show that the corresponding Cauchy problemhas a unique global solution. We then derive a numerical scheme based on an appropriate modificationof Yee's scheme, that we show to preserve some importantproperties of the continuous model such as the conservation of the normof the magnetization and the decay of the electromagnetic energy.Stability is proved under a suitable CFL condition.Some numerical results for the 1D model are presented.

We investigate time harmonic Maxwell equations in heterogeneous media, where thepermeability μ and the permittivity ε are piecewise constant. Theassociated boundary value problem can be interpreted as a transmission problem. Ina very natural way the interfaces can have edges and corners. We give a detaileddescription of the edge and corner singularities of the electromagnetic fields.