Muratbekov, M. and Igisinov, S. (2015) “On closability and bounded invertibility of mixed type differential operator in an unbounded domain”, Bulletin of the Karaganda University. Mathematics Series, 2(78), pp. 106–112. Available at: https://mts.buketov.edu.kz/mathematics-vestnik/article/view/30 (Accessed: 9 December 2025).