Ignat'ev, A. O. “On the Existence of a Lyapunov Function As a Quadratic Form for Impulsive Systems of Linear Differential Equations”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 62, no. 11, Nov. 2010, pp. 1451–1458,
https://umj.imath.kiev.ua/index.php/umj/article/view/2970.