Extremal covariant positive operator valued measures: the case of a   compact symmetry group
  Given a unitary representation U of a compact group G and a transitive G-space $\Omega$, we characterize the extremal elements of the convex set of all U-covariant positive operator valued measures. 
Claudio Carmeli, Teiko Heinosaari, Juha-Pekka Pellonp\"a\"a,   Alessandro Toigo