Referat na podstawie książki:
Dai, Feng, and Yuan Xu, Approximation theory and harmonic analysis on spheres and balls, New York: Springer, 2013
oraz pracy:
Lizorkin, P. I., and Kh. P. Rustamov, Nikol'skii-Besov spaces on the sphere in connection with approximation theory, Proceedings of the Steklov Institute of Mathematics-AMS Translation 204 (1994): 149-172.