THE TWO-SIDED ESTIMATES OF THE FREDHOLM RADIUS AND COMPACTNESS CONDITIONS FOR THE OPERATOR ASSOCIATED WITH A SECOND-ORDER DIFFERENTIAL EQUATION. JMMCS [Internet]. 2021 Sep. 27 [cited 2026 Jul. 13];110(2):18-26. Available from: https://bm.kaznu.kz/index.php/kaznu/article/view/938