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, [S. l.], v. 84, n. 4, p. 46–51, 2016. DOI: 10.31489/2016m4/46-51. Disponível em: http://mts.buketov.edu.kz/index.php/mathematics-vestnik/article/view/122. Acesso em: 23 aug. 2025.