SymокопV. G., and E. P. Trofimchuk. “On Regularity of Linear Systems With a Degenerate Matrix by the Derivative”. Ukrains’kyi Matematychnyi Zhurnal 45, no. 2 (February 25, 1993): 279–286. Accessed June 2, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5810.