One the masking problem for the two-dimensional Helmholtz equation

Control problems for 2-D Helmholtz equation in a bounded domain with mixed boundary conditions are considered. The boundary impedance entering into impedance boundary condition for the field plays the role of control. The solvability of both the direct problem and the control problem is proved. The uniqueness and stability of optimal solutions with respect to small perturbations of both the cost functional and a given function are established.