1. S. Agmon, Lectures on Elliptic Boundary Problems, Van Nostrand, New York, 1964.
2. T. Angell, D. Colton, and A. Kirsch, The three dimensional inverse scattering problem for acoustic waves, J. Diff. Eq. 46(1982), 46–58.
3. T. Angell, R. Kleinman, and G. Roach, An inverse transmission problem for the Helmholtz equation, Inverse Probl. 3(1987), 149–180.
4. A. M. Berthier, Spectral Theory and Wave Operators for the Schrödinger Equation, Pitman, Boston, 1982.
5. V. Buslaev, Scattered plane waves, spectral asymptotics, and trace formulas in exterior problems, Dokl. Akad. Nauk. SSSR 197(1971), 1067–1070.