Short talk: Restriction problems of Hilbert theta series

We consider the problem of whether the diagonal restrictions of a Hilbert theta series over a totally real number field span the whole space of elliptic modular forms. We transfer this problem to definite quaternionic Hilbert modular forms via theta correspondences and get the surjectivity of the restriction map by applying equidistribution properties of unipotent orbits in SL(2) as well as Jacquet-Langlands correspondences.