1. D. Gilbarg, N.S. Trudinger, Elliptic Partial Differential Equations of Second Order (Springer, Berlin, 1998)
2. P.A. Kuchment: Bull. (New Ser.) Am. Math. Soc. 53, 343 (2016); P.A. Kuchment: Floquet Theory For Partial Differential Equations, Operator Theory: Advances and Applications, vol. 60, (Birkhauser Verlag, Basel, 1993)
3. Y.E. Karpeshina, Perturbation Theory for the Schrödinger Operator with a Periodic Potential. Lecture Notes in Mathematics, vol. 1663 (Springer, Berlin, 1997)
4. O. Veliev, Multidimensional Periodic Schrödinger Operator: Perturbation Theory and Applications. Springer Tracts in Modern Physics, vol. 263 (Springer, Berlin, 2015)
5. P.Y. Yu, M. Cardona, Fundamentals of Semiconductors, 2nd edn. (Springer, Berlin, 1999)