1. D. McGarvey, “Operators commuting with translations by one. Part II. Differential operators with periodic coefficients in $$L_{p}(-\infty ,\infty)$$,” J. Math. Anal. Appl. 11, 564–596 (1965).
2. D. McGarvey, “Operators commuting with translations by one. Part III. Perturbation results for periodic differential operators,” J. Math. Anal. Appl. 12, 187–234 (1965).
3. D. McGarvey, “Linear differential systems with periodic coefficients involving a large parameter,” J. Diff. Eq. 2, 115–142 (1966).
4. F. S. Rofe-Beketov, “The spectrum of nonself-adjoint differential operators with periodic coefficients,” Sov. Math. Dokl. 4, 1563–1564 (1963).
5. O. A. Veliev, “Uniform convergence of the spectral expansion for a differential operator with periodic matrix coefficients,” Boundary Value Problems, Article ID 628973 (2008).