Zhestkov , S. V., and E. A. Ermolaev. “One Algebraic Method to Study Stability of Linear Normal Systems in Partial Derivatives”. Ukrains’kyi Matematychnyi Zhurnal, vol. 44, no. 4, Apr. 1992, pp. 480-9, https://umj.imath.kiev.ua/index.php/umj/article/view/7889.