Ospanov, K. N., & Yesbayev, A. N. (2021). THE TWO-SIDED ESTIMATES OF THE FREDHOLM RADIUS AND COMPACTNESS CONDITIONS FOR THE OPERATOR ASSOCIATED WITH A SECOND-ORDER DIFFERENTIAL EQUATION. Journal of Mathematics, Mechanics and Computer Science, 110(2), 18–26. https://doi.org/10.26577/JMMCS.2021.v110.i2.02