“…Viewed as functions 4>, they are piecewise polynomial functions which have nonnegative, nontrivial Fourier transform, when transformed as functions <f> m K"< This means, by a famous theorem of Bochner, that the interpolation matrices for interpolation at the centres are positive definite if the centres are distinct and lie in R\ Therefore they are called positive definite functions. For his purpose, Wu makes essential use of nonnegativity of the Fourier transform of <j>, if <j> is taken from a set of certain multiply monotone functions, see also [11,3]. Specifically, he creates positive definite radial functions <f> in one dimension first, and then uses a certain differentiation operator, applied to 4>, to lift these univariate functions that give rise to positive definite interpolation matrices to higher dimensions (and lower smoothness).…”