Igisinov, S.Zh., Zhumaliyeva, L.D., Suleimbekova, A.O., & Bayandiyev, Ye.N. (2022). Estimates of singular numbers (s-numbers) for a class of degenerate elliptic operators. Bulletin of the Karaganda University. Mathematics Series, 3(107), 51–58. https://doi.org/10.31489/2022m3/51-58