Generalizations of Schoenberg's theorem on positive definite kernels

Abstract: The seminal theorem of I.J. Schoenberg characterizes positive definite (p.d.) kernels on the unit sphere $S^{n-1}$ invariant under the automorphisms of the sphere. We obtain two generalizations of this theorem for p.d. kernels on fiber bundles. Our first theorem characterizes invariant p.d. kernels on bundles whose fiber is a product of a compact set and the unit sphere. This result implies, in particular, a characterization of invariant under the automorphisms of the sphere, p.d. kernels on a product of $S^{n-1}$ and a compact set. Our second result characterizes invariant p.d. kernels on the bundle whose fiber is $S^{n-1}$, base space is $(S^{n-1})^{r}$ and map is the projection on the base space. This set of kernels is isomorphic to the set of invariant under the automorphisms of the sphere continuous functions $F$ on $(S^{n-1})^{r+2}$ such that $F(\cdot,\cdot ,Z)$ is positive definite for every $Z \in (S^{n-1})^{r}$. When $Z$ is fixed, this class reduces to the class of p.d. kernels invariant under the stabilizer of $Z$ in the automorphism group of the sphere. For $r=1$ these kernels have been used to obtain upper bounds for the spherical codes problem. Our extension for $r>1$ can be used to construct new upper bounds on the size of spherical codes.