DragunovD. V., MakarovV. L., and SemberD. A. “FD-Method for Solving the Nonlinear Klein - Gordon Equation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 64, no. 10, Oct. 2012, pp. 1394-15, https://umj.imath.kiev.ua/index.php/umj/article/view/2667.