Б.А. Дуйсенгалиева, and А.С. Науразбекова. 2019. “An Example of a Non-Triangulable Derivation of a Differential Polynomial Algebra of Rank 2”. Bulletin of L.N. Gumilyov Eurasian National University. Mathematics, Computer Science, Mechanics Series 128 (3):53-57. https://bulmathmc.enu.kz/index.php/main/article/view/50.