In this talk, I will present a classic approach to prove the controllability of the scalar wave equation, including the Hilbert uniqueness method, microlocal defect measures, and unique continuation properties. In particular, I will focus on the properties of the defect measures and explain some ideas on the applications of the microlocal measures in the wave control problems. At the end, I will give a brief introduction on some related results on the controllability of wave systems.
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