Akhmetov, M.U. (1993) “On the smoothness of solutions of differential equations with a discontinuous right-hand side”, Ukrains’kyi Matematychnyi Zhurnal, 45(12), pp. 1587–1594. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/5965 (Accessed: 7 April 2025).