BugirM. K. (1992) “Study of properties of firmness of solutions of equations with partial derivatives by the method of separation of variables ”, Ukrains’kyi Matematychnyi Zhurnal, 44(3), pp. 317-323. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/8100 (Accessed: 27July2024).