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September 1, 2003
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In this paper we introduce and analyze a two-level Schwarz additive domain decomposition method used as a preconditioner of a GMRES algorithm for solving unsymmetric systems arising from the finite volume element or covolume methods for elliptic problems. The subproblems are solved by inexact solvers. We show that the method converges for both circumcentric and barycentric covolume methods. In the generous overlap case, the method is shown to be optimal, i.e., the conditioner number is uniformly bounded in coarse and fine mesh sizes.

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September 1, 2003
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In the preceding paper [24], a method is described for an explicit hierarchical (ℋ-matrix) approximation to the inverse of an elliptic differential operator with piecewise constant/smooth coefficients in ℝ d . In the present paper, we proceed with the ℋ-matrix approximation to the Green function. Here, it is represented by a sum of an ℋ-matrix and certain correction term including the product of data-sparse matrices of hierarchical formats based on the so-called boundary concentrated FEM [26]. In the case of jumping coefficients with respect to non-overlapping domain decomposition, the approximate inverse operator is obtained as a direct sum of local inverses over subdomains and the Schur complement inverse on the interface corresponding to the boundary concentrated FEM. Our Schur complement matrix provides the cheap spectrally equivalent preconditioner to the conventional interface operator arising in the iterative substructuring methods by piecewise linear finite elements.

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September 1, 2003
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We concentrate on a model diffusion equation on a Lipschitz simply connected bounded domain with a small diffusion coefficient in a Lipschitz simply connected subdomain located strictly inside of the original domain. We study asymptotic properties of the solution with respect to the small diffusion coefficient vanishing. It is known that the solution asymptotically turns into a solution of a corresponding diffusion equation with Neumann boundary conditions on a part of the boundary. One typical proof technique of this fact utilizes a reduction of the problem to the interface of the subdomain, using a transmission condition. An analogous approach appears in studying domain decomposition methods without overlap, reducing the investigation to the surface that separates the subdomains and in theoretical foundation of a fictitious domain, also called embedding, method, e.g., to prove a classical estimate that guaranties convergence of the solution of the fictitious domain problem to the solution of the original Neumann boundary value problem. On a continuous level, this analysis is usually performed in an H 1/2 norm for second order elliptic equations. This norm appears naturally for Poincaré-Steklov operators, which are convenient to employ to formulate the transmission condition. Using recent advances in regularity theory of Poincaré-Steklov operators for Lipschitz domains, we provide, in the present paper, a similar analysis in an H 1/2+α norm with α > 0, for a simple model problem. This result leads to a convergence theory of the fictitious domain method for a second order elliptic PDE in an H 1+α norm, while the classical result is in an H 1 norm. Here, α < 1/2 for the case of Lipschitz domains we consider.

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September 1, 2003
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The subject of the paper is the derivation and analysis of third order finite volume evolution Galerkin schemes for the two-dimensional wave equation system. To achieve this the first order approximate evolution operator is considered. A recovery stage is carried out at each level to generate a piecewise polynomial approximation Ũ n = R h U n ∈ from the piecewise constant U n ∈ , to feed into the calculation of the fluxes. We estimate the truncation error and give numerical examples to demonstrate the higher order behaviour of the scheme for smooth solutions.