VlasenkoL. A., Piven’A. L., and RutkasA. G. 2004. “Criteria for the Well-Posedness of the Cauchy Problem for Differential Operator Equations of Arbitrary Order”. Ukrains’kyi Matematychnyi Zhurnal 56 (11), 1484-1500. https://umj.imath.kiev.ua/index.php/umj/article/view/3860.