Aitzhanov, S. E., Bekenaeva К. S., and Zhumagul G. О. “Solvability of a Pseudohyperbolic Equation With a Nonlinear Boundary Condition”. Journal of Mathematics, Mechanics and Computer Science, vol. 108, no. 4, Dec. 2020, pp. 26-37, doi:10.26577/JMMCS.2020.v108.i4.03.