Yeskermessuly, A. “Investigation of the Deficiency Indices of the Minimum Two-Term Fourth-Order Differential Operator With Irregular Coefficients”. Bulletin of the Karaganda University. Mathematics Series 84, no. 4 (December 30, 2016): 46–51. Accessed August 23, 2025. http://mts.buketov.edu.kz/index.php/mathematics-vestnik/article/view/122.