Ring structures for holomorphic discrete series and Rankin-Cohen   brackets
  In the present note we discuss two different ring structures on the set of holomorphic discrete series of a causal symmetric space of Cayley type $G/H$ and we suggest a new interpretation of Rankin-Cohen brackets in terms of intertwining operators arising in the decomposition of tensor products of holomorphic discrete series representations. 
Gerrit van Dijk, Michael Pevzner