Kharibegashvili, S., and B. Midodashvili. 2015. “On the Solvability of a Problem Nonlocal in Time for a Semilinear Multidimensional Wave Equation”. Ukrains’kyi Matematychnyi Zhurnal 67 (1), 88–105. https://umj.imath.kiev.ua/index.php/umj/article/view/1965.