Б.А. Дуйсенгалиева, and А.С. Науразбекова. “An Example of a Non-Triangulable Derivation of a Differential Polynomial Algebra of Rank 2”. BULLETIN OF THE L.N. GUMILYOV EURASIAN NATIONAL UNIVERSITY. Mathematics. Computer Science. Mechanics Series, vol. 128, no. 3, Sept. 2019, pp. 53-57, https://bulmathmc.enu.kz/index.php/main/article/view/50.