“The Hypoellipticity and Parametrices for a Class of Nonprincipal Type LPDOs”, J. Part. Diff. Eq., vol. 6, no. 4, pp. 339–348, May 2020, Accessed: Dec. 05, 2025. [Online]. Available: https://global-sci.com/index.php/jpde/article/view/3755