Quaternion $H$-type group and differential operator $\Delta_{\lambda}$
  We study the relations between the quaternion $H$-type group and the boundary of the unit ball on two dimensional quaternionic space. The orthogonal projection of the space of square integrable functions defined on quaternion $H$-type group into its subspace of boundary values of $q$-holomorphic functions is consider. The precise form of Cauchy-Szego kernel and the orthogonal projection operator is obtained. The fundamental solution for the operator $\Delta_{\lambda}$ is found. 
Der-Chen Chang, Irina Markina