[1] Samoilenko A. M. M., Ronto N. I. Numerical and analytical methods of investigation of periodic solutions. - K. : Vyshcha Shkola, 1976. - 184 p.

[2] Samoilenko A. M., Khoma-Mohylska S. G. Analytical method for finding 2π -periodic solutions of hyperbolic equations of the second order // Doklady. NAS of Ukraine. - 2010. - N4. - P. 25-29.

[3] Samoilenko A. M., Khoma N. G., Khoma-Mohylska S. G. A special case of the existence of 2π-periodic solutions of boundary value problems for a hyperbolic equation of the second order // Doklady. NAS of Ukraine. - 2012. - N2. - P. 35-41.

[4] Mitropolskyi Y. O., Khoma-Mohylska S. H. Conditions for the existence of solutions of a boundary value problem for a nonuniform linear hyperbolic equation of the second order. I // Ukr. math. zhurnal - 2005. - 57, N7. - p. 912-921.

[5] Mitropolsky Yu. Solutions of a boundary value problem for a nonuniform linear hyperbolic equation of the second order // Doklady NASU. NAS of Ukraine. - 2008. - N7. - P. 30-36.

[6] Rabinowitz P. Periodic solutions of hyperbolic partial differential equations //Comm. Pure Appl. Math. – 1967. – 20 , № 1. – Р. 145-205.

[7] Ptashnik B. I. Uncorrected boundary value problems for partial differential equations. - K. : Naukova Dumka, 1984. -264 p.

[8] Ptashnyk B. Y., Ilkiv V. S., Kmit I. Y., Polischuk V. M. Nonlocal boundary value problems for partial differential equations - K. : Naukova Dumka, 2002. - 416 p.