1. B. I. Ptashnik, Ill-Posed Boundary-Value Problems for Partial Differential Equations (“Naukova Dumka”, Kiev, 1984) [in Russian].
2. B. I. Ptashnik, V. S. Il’kiv, I. Ya. Kmit’, and V. M. Polishchuk, Nonlocal Boundary-Value Problems for Partial Differential Equations (“Naukova Dumka”, Kiev, 2002) [in Ukrainian].
3. A. M. Nakhushev, Problems with Shifts for Partial Differential Equation (Nauka, Moscow, 2006) [in Russian].
4. T. Kiguradze and V. Lakshmikantham, “On the Dirichlet problem for fourth-order linear hyperbolic equations,” Nonlinear Anal. 49 (2), 197–219 (2002).
5. B. Midodashvili, “A nonlocal problem for fourth order hyperbolic equations with multiple characteristics,” Electron. J. Differential Equations 2002 (85), 1–7 (2002).