- Kh. P. Rustamov, “Equivalence of K-functional and modulus of smoothness of functions on the sphere”, Math Notes, 52, no. 3, 1992,
965 
- Bao-huai Sheng, “On Approximation by Reciprocals of Spherical Harmonics in L p Norm”, Acta Mathematicae Applicatae Sinica, English Series, 21, no. 4, 2005,
529