Approximation of unbounded operators by bounded operators and related extremal problems

Abstract

Contents §0. Introduction §1. Stechkin's bound for the degree of best approximation of an operator §2. Connection between Stechkin's problem and the ill-posed problem of recovering the values of an unbounded operator §3. Best approximation of functionals §4. Best approximation of differentiation operators and inequalities between norms of derivatives for functions of one variable on the line and half-line §5. Approximation of differentiation operators on classes of functions of several variables §6. Approximation of invariant operators §7. Approximation of one class of differentiable functions by another class of smoother functions Bibliography