1. V. Barbu, Analysis and Control of Nonlinear Infinite-Dimensional Systems, Academic Press, Boston, 1993.
2. V. Barbu, N. Pavel, Periodic optimal control in Hilbert space, Appl. Math. Optim., 33 (1996), 169–188.
3. V. Barbu, N. Pavel, An inverse problem for the one-dimensional wave equation (to appear).
4. H. Brézis, Periodic solutions of nonlinear vibrating string and duality principles, Bull. Amer. Math. Soc., 8 (1983), 409–426.
5. H. Brézis, L. Nirenberg, Forced vibrations for nonlinear wave equations, Comm. Pure Appl. Math., 31 (1978), 2–30.