1. S. Alinhac, Non-unicité du problème de Cauchy. Ann. Math. 117, 77–108 (1983)
2. M. Belishev, An approach to multidimensional inverse problems for the wave equation. Dokl. Akad. Nauk SSSR 297, 524–527 (1987). Engl. transl. Soviet Math. Dokl. 36, 481–484 (1988)
3. M. Belishev, V. Kurylev, To the reconstruction of a Riemannian manifold via its spectral data (BC-method). Commun. Partial Differ. Equ. 17, 767-804 (1992)
4. A. Blagoveščenskii, A one-dimensional inverse boundary value problem for a second order hyperbolic equation. Zap. Nauchn. Sem. LOMI 15, 85–90 (1969) (in Russian)
5. A. Blagoveščenskii, Inverse boundary problem for the wave propagation in an anisotropic medium. Trudy Mat. Inst. Steklova 65, 39–56 (1971) (in Russian)