Orthogonal expansions related to compact Gelfand pairs

Abstract: Given a compact Gelfand pair (G,K) and a locally compact group L, we characterize the class P_K^\sharp(G,L) of continuous positive definite functions f:G\times L\to \C which are bi-invariant in the G-variable with respect to K. The functions of this class are the functions having a uniformly convergent expansion \sum_{\varphi\in Z} B(\varphi)(u)\varphi(x) for x\in G,u\in L, where the sum is over the space Z of positive definite spherical functions \varphi:G\to\C for the Gelfand pair, and (B(\varphi)){\varphi\in Z} is a family of continuous positive definite functions on L such that \sum{\varphi\in Z}B(\varphi)(e_L)<\infty. Here e_L is the neutral element of the group L. For a compact abelian group G considered as a Gelfand pair (G,K) with trivial K={e_G}, we obtain a characterization of P(G\times L) in terms of Fourier expansions on the dual group \widehat{G}. The result is described in detail for the case of the Gelfand pairs (O(d+1),O(d)) and (U(q),U(q-1)) as well as for the product of these Gelfand pairs. The result generalizes recent theorems of Berg-Porcu (2016) and Guella-Menegatto (2016)