First order hyperbolic equations with constant operator coefficients

This paper considers the Cauchy problem for a hyperbolic equation with constant operator coefficients in Hilbert space:
$$\frac{\partial\psi}{\partial t}=\sum_{k=1}^n A_k\frac{\partial\psi}{\partial x_k}+B\psi,$$
where $A_k$ and $B$ are selfadjoint operators and $B$ is semibounded.
As an example we consider ultraparabolic systems.
Bibliography: 10 titles.