NasykhovaL. V., and TerekhinM. T. “The Existence of a Bifurcation Value of a Parameter of a System of Differential Equations With Deviating Argument”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 49, no. 6, June 1997, pp. 799–805, https://umj.imath.kiev.ua/index.php/umj/article/view/5064.