Uniform polynomial approximation with A⁎ weights having finitely many zeros

Abstract

We prove matching direct and inverse theorems for uniform polynomial approximation with A⁎ weights (a subclass of doubling weights suitable for approximation in the L∞ norm) having finitely many zeros and not too “rapidly changing” away from these zeros. This class of weights is rather wide and, in particular, includes the classical Jacobi weights, generalized Jacobi weights and generalized Ditzian–Totik weights. Main part and complete weighted moduli of smoothness are introduced, their properties are investigated, and equivalence type results involving related realization functionals are discussed.