“…The first attempt for realizing this idea was carried out in [13,14] for a system of wave equations with Dirichlet boundary condition by incomplete Neumann observations. Later, this idea was used in [11,19] for Neumann and Robin boundary conditions, and further developed in [18,21] for an elliptic system with Neumann boundary conditions observed by incomplete Dirichlet observations. Additionally, the coupling matrix A should be nilpotent (see [2,14,16]), or symmetric and close to a scalar matrix etc.…”