A Survey of Weighted Approximation for Exponential Weights (original) (raw)

2007, Eprint Arxiv Math 0701099

Let W : R → (0, 1] be continuous. Bernstein's approximation problem, posed in 1924, deals with approximation by polynomials in the weighted uniform norm f → f W L∞(R). The qualitative form of this problem was solved by Achieser, Mergelyan, and Pollard, in the 1950's. Quantitative forms of the problem were actively investigated starting from the 1960's. We survey old and recent aspects of this topic, including the Bernstein problem, weighted Jackson and Bernstein Theorems, Markov-Bernstein and Nikolskii inequalities, orthogonal expansions and Lagrange interpolation. We present the main ideas used in many of the proofs, and different techniques of proof, though not the full proofs. The class of weights we consider is typically even, and supported on the whole real line, so we exclude Laguerre type weights on [0, ∞). Nor do we discuss Saff's weighted approximation problem, nor the asymptotics of orthogonal polynomials. MSC: 41A10, 41A17, 41A25, 42C10 Keywords: Weighted approximation, polynomial approximation, Jackson-Bernstein theorems Contents 1 Bernstein's Approximation Problem 2 Some Ideas for the Resolution of Bernstein's Problem 3 Weighted Jackson and Bernstein Theorems 4 Methods for Proving Weighted Jackson Theorems 19 4.