Zarubin, A. N. 1997. “Integral Representation of a Solution of the Cauchy Problem for a Degenerating Hyperbolic Equation With Retarded Argument”. Ukrains’kyi Matematychnyi Zhurnal 49 (10), 1332–1336. https://umj.imath.kiev.ua/index.php/umj/article/view/5131.