An Unified Approach to Approximation Theorems of Dini-Lipschitz-Type

Abstract:

      This survey paper studies the approximation of (polynomial) processes for which the operator norms do not form a bounded sequence. In view of familiar direct estimates and quantitative uniform boundedness principles, a unified approach is given to results concerning the equivalence of Dini-Lipschitz-type conditions with (strong) convergence on (smoothness) classes. Emphasis is laid upon the necessity of these conditions, essential ingredients of the proofs are suitable modifications of the familiar gliding hump method. Apart from the classical results concerned with Fourier partial sums, explicit applications are treated for (trigonometric as well as algebralc) Lagrange interpolation, interpolatory quadrature rules based upon Jacobl knots, multipliers or strong convergence, and for Bochner-Riesz means of multivariate Fourier series for parameter values below the critical index.