L. P. Nizhnik and L. A. . Taraborkin, “On self-conjugate operators generated nonhomogeneous elliptical problems with discontinuity boundary conditions and conditions of conjugacy”, Ukr. Mat. Zhurn., vol. 43, no. 3, pp. 374–381, Feb. 1991, Accessed: Jan. 12, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/9618