B. I. Ptashnik and M. M. Symotyuk, “Multipoint Problem for Nonisotropic Partial Differential Equations with Constant Coefficients”, Ukr. Mat. Zhurn., vol. 55, no. 2, pp. 241–254, Feb. 2003, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3902