1. O. A. Ladyzhenskaya , Boundary Value Problems of Mathematical Physics, Springer-Verlag, New York etc. (1985).
2. M. Sh. Birman and M. Z. Solomyak, Spectral Theory of Selfadjoint Operators in Hilbert Space [in Russian], Leningrad Univ., Leningrad (1980).
3. A. V. Sobolev and J. Walthoe, “Absolute continuity in periodic waveguides,” Proc. London Math. Soc., 85, No. 1, 717–741 (2002).
4. T. A. Suslina and R. G. Shterenberg, “Absolute continuity of the spectrum of the magnetic Schrödinger operator with a metric in a two-dimensional periodic waveguide,” Algebra Analiz, 14:2, 159–206 (2002).
5. I. Kachkovskii and N. Filonov, “Absolute continuity of the spectrum of a periodic Schrödinger operator in a multidimensional cylinder,” Algebra Analiz, 21:1, 133–152 (2009).