THE TWO-SIDED ESTIMATES OF THE FREDHOLM RADIUS AND COMPACTNESS CONDITIONS FOR THE OPERATOR ASSOCIATED WITH A SECOND-ORDER DIFFERENTIAL EQUATION. ҚазҰУ Хабаршысы. Математика, механика, информатика сериясы, [S. l.], v. 110, n. 2, p. 18–26, 2021. DOI: 10.26577/JMMCS.2021.v110.i2.02. Disponível em: https://bm.kaznu.kz/index.php/kaznu/article/view/938. Acesso em: 5 jul. 2026.