[E.1]

G. Eskin,*Paramatrix and propagation of singularities for the interior mixed hyperbolic problem*, J. Analyse Math.,**32** (1977), pp. 17–62.

[E.2]

G. Eskin,*Initial-boundary value problems for second order hyperbolic equations with general boundary conditions - I, II*, J. Analyse Math.,**40** (1981), pp. 43–89, and Communic. in P.D.E.,**10** (1985), pp. 1117–1212.

[H.1]

L.Hormander,*The analysis of linear partial differential operators, I, II, III, IV*, Springer-Verlag, 1983, 1983, 1985, 1985.

[L.1]

I.Lasiecka,*Sharp regularity results for mixed hyperbolic problems of second order*, Lectures Notes in Mathematics, 1223, Springer-Verlag, 1986.

[L.2]

J. L. Lions,*Contrôle des systèmes distribués singuliers*, Gauthier-Villars, Paris, 1983.

[L-T.1]

I.Lasiecka - R.Triggiani,*A cosine operator approach to modeling L*
_{2}(0,*T;L*
_{2}(*Γ*))*boundary input hyperbolic equations*, Appl. Math. & Optimiz., i (1981), pp. 35–83.

[L-T.2]

I. Lasiecka -R. Triggiani,*Regularity of hyperbolic equations under* L_{2}(0,*T; L*
_{2}(*Γ*))-*boundary terms*, Appl. Math. & Optimiz.,**10** (1983), pp. 275–286.

[L-T.3]

I. Lasiecka -R. Triggiani,*Trace regularity of the solutions of the wave equation with homogeneous Neumann boundary conditions and compactly supported data*, J. Math. Anal. & Appl.,**141**, no. 1 (1989), pp. 49–71.

[L-T.4]

I. Lasiecka -R. Triggiani,*A lifting theorem for the time regularity of solutions to abstract equations with unbounded operators and applications to hyperbolic equations*, Proc. Amer. Math. Soc.,**104**, no. 3 (1988), pp. 745–755.

[L-T.5]

I.Lasiecka - R.Triggiani,*Regularity theory of hyperbolic equations with non-homogeneous Neumann boundary conditions*, Part II:*General boundary data*, preprint 1989.

[L-T.6]

I.Lasiecka - R.Triggiani,*Differential Riccati Equations with unbounded coefficients: applications to boundary control/boundary observation hyperbolic problems*, preprint 1990.

[L-T.7]

I.Lasiecka - R.Triggiani,*Announcement* in Atti Acc. Lincei Rend. Fis., (8), LXXXIII (1989), to appear.

[L-L-T.1]

I. Lasiecka -J. L. Lions -R. Triggiani,*Nonhomogeneous boundary value problems for second order hyperbolic operators*, J. Math. Pures et Appl.,**69** (1986), pp. 149–192.

[L-M.1]

J. L.Lions - E.Magenes,*Nonhomogeneous Boundary Value Problems and Applications*, I, II (1972) and III (1973), Springer-Verlag.

[M.1]

S. Myatake,*Mixed problem for hyperbolic equations of second order*, J. Math. Kyoto Univ.,**130**, no. 3 (1973), pp. 435–487.

[S.1]

R. Sakamoto,*Mixed problems for hyperbolic equations, I, II*, J. Math. Kyoto Univ.,**10**, no. 2 (1970), pp. 343–373 and**10**, no. 3 (1970), pp. 403–417.

[S.2]

R.Sakamoto,*Hyperbolic boundary value problems*, Cambridge University Press (1982).

[S.3]

W. W. Symes,*A trace theorem for solutions of the wave equation and the remote determination of acoustic sources*, Math. Methods in the Applied Sciences,**5** (1983), pp. 35–93.

[T.1]

M. E. Taylor,*Pseudo-differential operators*, Princeton University Press, Princeton, New Jersey (1981).

[T.2]

R.Triggiani,*An announcement of sharp regularity theory for second order hyperbolic equations of Neumann type*, Springer-Verlag Lecture Notes, in*Control and Information Sciences*,**114**, pp. 284–288; Proceedings IFIP Conference on optimal control for systems governed by partial differential equations, held at University of Santiago de Campostela, Spain, July 1987