Kanguzhin, B. E., and D. B. Nurakhmetov. “Correct Boundary Value Problems for 2-Order in Nonhomogeneous Differential Equation With Variable Coefficients”. Journal of Mathematics, Mechanics and Computer Science, vol. 64, no. 1, Feb. 2019, pp. 9-26, https://bm.kaznu.kz/index.php/kaznu/article/view/590.