1. V. A. Marchenko, The Sturm-Liouville Operators and Their Applications [in Russian], Naukova Dumka, Kiev, 1977.
2. B. M. Levitan, The Inverse Sturm-Liouville Problem [in Russian], Nauka, Moscow, 1984.
3. I. M. Gel’fand and G. E. Shilov, Generalized Functions and Actions over Them [in Russian], vol. 1, Fizmatlit, Moscow, 1959.
4. A. M. Savchuk and A. A. Shkalikov, “The Sturm-Liouville operators with singular potentials,” Mat. Zametki [Math. Notes], 66 (1999), no. 6, 897–912.
5. A. M. Savchuk and A. A. Shkalikov, “The Sturm-Liouville operators with distribution potentials,” Trudy Moskov. Mat. Obshch. [Trans. Moscow Math. Soc.], 64 (2003), 159–219.