Interpolation of Spaces of Functions of Positive Smoothness on a Domain

Interpolation spaces are described for spaces of functions of positive smoothness on a domain $G$ of the Euclidean space $\mathbb R^n$ that satisfies the flexible cone condition. As a consequence, multiplicative estimates for the norms of functions are obtained. The arguments are based on integral representations of functions over a flexible cone in terms of the local approximations of functions by polynomials and on estimates of the arising convolution operators.