1. I. V. Saveliev, The Course of General Physics, Vol. 4: Waves. Optics (AST Astrel, Moscow, 2008) [in Russian].
2. I. M. Babakov, Oscillation Theory (Drofa, Moscow, 2004) [in Russian].
3. A. N. Tikhonov and A. A. Samarsky, Equations of Mathematical Physics (Izd-vo MGU, Moscow, 1999) [in Russian].
4. K. S. Kabisov, T. F. Kamalov, and V. A. Lurie, Oscillations and Wave Processes: Theory. Tasks with Solutions (KomKniga, Moscow, 2010) [in Russian].
5. I. V. Kudinov and V. A. Kudinov, “Exact Closed-Form Solution of the Hyperbolic Equation of String Vibrations with Material Relaxation Properties Taken into Account,” Izv. Akad. Nauk. Mekh. Tv. Tela, No. 5, 64–76 (2014) [Mech. Sol. (Engl. Transl.) 49 (5), 531–542 (2014)].