1. F. S. Rofe-Beketov and A. M. Khol’kin, Spectral Analysis of Differential Operators (Mariupol, 2001) [in Russian].
2. M. A. Naimark, Linear Differential Operators (Nauka, Moscow, 1969) [in Russian].
3. A. M. Savchuk and A. A. Shkalikov, “Sturm-Liouville Operators with Potential Distributions,” Trudy Mosk. Mat. Obshchestva 64, 159–212 (2003).
4. A. A. Vladimirov, “On the Convergence of Sequences of Ordinary Differential Operators,” Mat. Zametki 75, 941–943 (2004).
5. P. A. Binding and H. Volmer, “Oscillation Theory for Sturm-Liouville Problems with Indefinite Coefficients,” Proc. Roy. Soc. Edinburg 131 989–1002 (2001).