KovachY. I. “Theorem of the «fork» in Cauchy’s Problem for Nonlinear Partial Differential Equations of Higher Orders”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 18, no. 5, Aug. 1966, pp. 23-35, https://umj.imath.kiev.ua/index.php/umj/article/view/8438.