Extending functions from nonisotropic Nikolskii-Besov spaces and approximating their derivatives (with supplement)

Abstract

The article examines nonisotropic Nikolskii and Besov spaces with norms defined using $L_p$-averaged moduli of continuity of functions of appropriate orders along the coordinate directions, instead of moduli of continuity of known orders for derivative functions along the same directions. The author builds continuous linear mappings of such spaces of functions defined in domains of certain type to ordinary nonisotropic Nikolskii and Besov spaces in $\mathbb{R}^d $ that are function extension operators, thus incurring coincidence of both kinds of spaces in the said domains. The article also provides weak asymptotics of approximation characteristics related to the problem of derivative reconstruction from function values at a given number of points, the S.B.Stechkin's problem for differential operator, and the problem of width asymptotics for nonisotropic Nikolskii-Besov classes in those domains. The supplement provides a broad-sense definition of $α$-type domain. It is demonstrated that all of the earlier statements established by the author with respect to the tasks of function extension, derivative reconstruction from function values, differential operator approximation with bounded operators, and width asymptotics for nonisotropic Nikolskii-Besov function classes defined in $α$-type domains (in a narrow sense), remain valid in the same formulation even for an $α$-type domain as construed in a broad sense.