Exact boundary value controllability in hyperbolic problems

Control processes which can be modeled by certain linear hyperbolic, partial differential equations are considered with the purpose of identifying those which can be exactly controlled to a specified state in some finite time by control forces applied on the boundary of the region ?? in which the process evolves. A class of processes for which such control is possible is determined in terms of asymptotic behavior of the fundamental solution of the modeling equation, and the optimal control time is shown to be no greater than the diameter of ??.