1. W. N. Everitt and L. Marcus, Boundary Value Problems and Symplectic Algebra for Ordinary Differential and Quasi-Differential Operators, Mathematical Surveys and Monographs, Vol. 61 (American Mathematical Society, 1999).
2. J. Eckhardt, F. Gestezy, R. Nichols, and G. Teschl, “Weyl–Titchmarsh theory for Sturm–Liuville operators with distributional potentials,” Opuscula Math. 33, 467–563 (2013).
3. W. N. Everitt and D. Race, “The regular representation of singular second order differential expressions using quasi-derivatives,” Proc. London Math. Soc. s3-65, 383–404 (1992). https://doi.org/10.1112/plms/s3-65.2.383
4. X. Lv, J. Ao, and A. Zettl, “Dependence of eigenvalues of fourth-order differential equations with discontinuous boundary conditions on the problem,” J. Math. Anal. Appl. 456, 671–685 (2017). https://doi.org/10.1016/j.jmaa.2017.07.021
5. Bao. Qinglan, Sun. Jiong, Hao. Xiaoling, and A. Zettl, “Characterization of self-adjoint domains for regular even order C-symmetric differential operators,” Electron. J. Qualitative Theory Differ. Equations 62, 1–17 (2019).