On the Derivatives of Radial Positive Definite Functions

Abstract A radial positive definite function ϕ(‖ · ‖) on R d which is 2k times differentiable at zero has 2k + [(d − 1)/2] continuous derivatives away from zero, and the estimate is sharp. Furthermore, ϕ(iv)(0) ≥ 3d|ϕ″(0)|2/(d + 2) if the derivatives exist; this proves a conjecture of Paul Switzer and has implications on the smoothness of isotropic random fields and their local averages. The techniques used have evolved in geostatistics, and side results refer to Matheron's turning bands operator and the montee.