Ospanov К. ., and Moldagali Е. . “CORRECTNESS CONDITIONS FOR FOURTH ORDER DIFFERENTIAL EQUATIONS WITH VARIABLE COEFFICIENTS”. Bulletin of Abai KazNPU. Series of Physical and Mathematical Sciences, vol. 90, no. 2, June 2025, pp. 116-23, doi:10.51889/2959-5894.2025.90.2.010.