1. Duistermatt, J.J. and L. Hörmander, Fourier integral Operators II, Acta Math. 128 (1972) 183–269.
2. Hörmander, L., Uniqueness theorems and wave front sets, C.P.A.M. 24 (1971) 79–183.
3. — The analysis of linear partial differential operators III, Berlin, Springer Verlag 1985.
4. Lagnese, J., Boundary value control of a class of hyperbolic equations in a general domain, SIAM J. Control and Optimization 15, (1977) 973–983.
5. Littman, W., Boundary control theory for hyperbolic and parabolic equations with constant coefficients, Annali Sc. N. Sup. Pisa Ser. IV 5, (1978) 567–580.