Kolmogorov problem in W r H ω [ 0 , 1 ] and extremal Zolotarev ω-splines [Book] Bagdasarov Sergey K. (1998)
Kolmogorov type inequalities for hypersingular integrals with homogeneous characteristic. Babenko, Vladislav F., Churilova, Mariya S. (2007) Banach Journal of Mathematical Analysis [electronic only]