Blending Approximations with Sine Functions.- Quasi-interpolation in the Absence of Polynomial Reproduction.- Estimating the Condition Number for Multivariate Interpolation Problems.- Wavelets on a Bounded Interval.- Quasi-Kernel Polynomials and Convergence Results for Quasi-Minimal Residual Iterations.- Rate of Approximation of Weighted Derivatives by Linear Combinations of SMD Operators.- Approximation by Multivariate Splines: an Application of Boolean Methods.- Lm, ?, s-Splines in ?d.- Constructive Multivariate Approximation via Sigmoidal Functions with Applications to Neural Networks.- Spline-Wavelets of Minimal Support.- Necessary Conditions for Local Best Chebyshev Approximations by Splines with Free Knots.- C1 Interpolation on Higher-Dimensional Analogs of the 4-Direction Mesh.- Tabulation of Thin Plate Splines on a Very Fine Two-Dimensional Grid.- The L2-Approximation Orders of Principal Shift-Invariant Spaces Generated by a Radial Basis Function.- A Multi-Parameter Method for Nonlinear Least-Squares Approximation.- Analog VLSI Networks.- Converse Theorems for Approximation on Discrete Sets II.- A Dual Method for Smoothing Histograms using Nonnegative C1-Splines.- Segment Approximation By Using Linear Functionals.- Construction of Monotone Extensions to Boundary Function

The first investigations of nonlinear approximation problems were made by P.L. Chebyshev in the last century, and the entire theory of uniform approxima tion is strongly connected with his name. By making use of his ideas, the theories of best uniform approximation by rational functions and by polynomials were developed over the years in an almost unified framework. The difference between linear and rational approximation and its implications first became apparent in the 1960's. At roughly the same time other approaches to nonlinear approximation were also developed. The use of new tools, such as nonlinear functional analysis and topological methods, showed that linearization is not sufficient for a complete treatment of nonlinear families. In particular, the application of global analysis and the consideration of flows on the family of approximating functions intro duced ideas which were previously unknown in approximation theory. These were and still are important in many branches of analysis. On the other hand, methods developed for nonlinear approximation prob lems can often be successfully applied to problems which belong to or arise from linear approximation. An important example is the solution of moment problems via rational approximation. Best quadrature formulae or the search for best linear spaces often leads to the consideration of spline functions with free nodes. The most famous problem of this kind, namely best interpolation by poly nomials, is treated in the appendix of this book."

During the week of December 8-13, 1984, a conference on Multi-Grid Methods was held at the Mathematisches Forschungs- institut (Mathematical Research Institute) in Oberwolfach. The conference was suggested by the GAMM-Committee "Effiziente numerische Verfahren fUr partielle Differentialgleichungen". We were pleased to have 42 participants from 12 countries. These proceedings contain some contributions to the confe- rence. The centre of interest in the more theoretical contri- butions were exact convergence proofs for multi-grid method. Here, the theoretical foundation for the application of the method to the Stokes equations, the biharmonic equation in its formulation as a mixed finite element problem and other more involved problems were investigated. Moreover, improvements and new attacks for getting quantitative results on convergence rates were reported. Another series of contributions was concerned with the de- velopment of highly efficient and fast algorithms for various partial differential equations. Also in this framework, the Stokes and the biharmonic equations were investigated. Other lectures treated problems from fluid mechanics (as Navier- Stokes and Euler equations), the dam-problem and eigenvalue problems. The editors would like to thank Professor M. Barner, the director of Mathematisches Forschungsinstitut Oberwolfach for making this conference possible. D. Braess, Bochum W. Hackbusch, Kiel U. Trottenberg, St. Augustin v CONTENTS O. AXELSSON: A mixed variable finite element method for the efficient solution of nonlinear diffusion and pot- tial flow equations ................................... .

This definitive introduction to finite element methods has been thoroughly updated for a third edition which features important new material for both research and application of the finite element method. The discussion of saddle-point problems is a highlight of the book and has been elaborated to include many more nonstandard applications. The chapter on applications in elasticity now contains a complete discussion of locking phenomena. The numerical solution of elliptic partial differential equations is an important application of finite elements and the author discusses this subject comprehensively. These equations are treated as variational problems for which the Sobolev spaces are the right framework. Graduate students who do not necessarily have any particular background in differential equations, but require an introduction to finite element methods will find this text invaluable. Specifically, the chapter on finite elements in solid mechanics provides a bridge between mathematics and engineering.

Bei der numerischen Behandlung partieller Differentialgleichungen treten oft uberraschende Phanomene auf. Neben der zugigen Behandlung der klassischen Theorie, die bis an die aktuelle Forschung heranfuhrt, wird deshalb viel Wert auf die Darstellung von Beispielen und Gegenbeispielen gelegt. Die Beispiele haben mit dazu beigetragen, dass das Buch jetzt zu den Standardwerken bei den Finiten Elementen zahlt.

Mit der funften Auflage erfolgte eine weitere Abrundung bei den Themen, deren Bedeutung in den letzten Jahren gewachsen ist. Mit der Theorie der a posteriori Fehlerschatzer wird a priori Information uber den Diskretisierungsfehler gewonnen, die in der klassischen Theorie noch nicht hergeleitet wurden und die scharfer als sonst eine Eigenart von a posteriori Schatzern beleuchtet. Die Behandlung von Platten in der Festkorpermechanik erhalt jetzt mit dem Zwei-Energien-Prinzip eine solide Grundlage, nachdem in der letzten Auflage die Behandlung von Locking Effekten in eine vollstandige Theorie mundete.

Das Buch richtet sich an Studierende der Mathematik im 3. Und 4. Studienjahr und in den spateren Kapiteln auch an junge Forscher, bei denen Finite Elemente im Mittelpunkt ihrer Arbeit stehen.

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