Operator valued positive definite kernels and differentiable universality

Abstract

We present a characterization for a positive definite operator valued kernel to be universal or $C_0$-universal, and apply these characterizations to a family of operator valued kernels that are shown to be well behaved. Later, we obtain a characterization for an operator valued differentiable kernel to be $C^q$-universal and $C_0^q$-universal. In order to obtain such characterization and examples we generalize some well known results concerning the structure of differentiable kernels to the operator valued context. On the examples is given an emphasis on the radial kernels on Euclidean spaces.