Kovalev, A. M., N. V. Kravchenko, and V. N. Nespirnyi. “Necessary Condition for the Stabilizability of Nonlinear Systems With Respect to a Part of Variables in the Class of Discontinuous Controls”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 58, no. 10, Oct. 2006, pp. 1434–1440, https://umj.imath.kiev.ua/index.php/umj/article/view/3544.