1. E. C. Titchmarsh, Eigenfunctions expansion associated with second order differential equations, London: Oxford University Press, 1962.
2. M. A. Naimark, Linear differential operators, part 2, London: Harrap, 1968.
3. Z. J. Cao, Ordinary differential operator (in Chinese), Beijing: Science Press, 1986.
4. A. Zettl, Sturm-Liouville theory, Providence, Rhode Island: American Mathematics Society, Mathematical Surveys and Monographs, 2005. http://dx.doi.org/10.1090/surv/121
5. A. P. Wang, Research on weimann conjecture and differential operators with transmission conditions (in Chinese), Ph. D. thesis, Inner Mongolia University, 2006, 66–74.