VitrychenkoI. E. 1994. “On Stability of the Trivial Solution of a Nonautonomous Quasilinear System Whose Characteristic Equation Has Multiple Roots”. Ukrains’kyi Matematychnyi Zhurnal 46 (8), 1072–1079. https://umj.imath.kiev.ua/index.php/umj/article/view/5660.