2021. 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. 110, 2 (Sep. 2021), 18–26. DOI:https://doi.org/10.26577/JMMCS.2021.v110.i2.02.