1. Danford, N., Schwartz, J.T.: Linear Operators, Part II: Spectral Theory, New York. Wiley-Interscience, USA (1988)
2. Gelfand, I.M.: Expansion in series of eigenfunctions of an equation with periodic coefficients. Soviet Math. Doklady 73, 1117–1120 (1950)
3. Kato, T.: Perturbation Theory for Linear Operators, Berlin. Springer-Verlag, Germany (1980)
4. McGarvey, D.C.: Differential operators with periodic coefficients in $$L_{p}(-\infty ,\infty )$$. J. Math. Anal. Appl. 11, 564–596 (1965)
5. Naimark, M.A.: Linear Differential Operators, London. George G. Harap & Company, England (1967)