Potapov, M. K., Berisha, F. M., “Approximation of classes of functions defined by a generalized $k$th modulus of smoothness”, East J. Approx., 4:2 (1998), 217–241
Potapov, M. K., Berisha, F. M., “Direct and inverse theorems of approximation theory for a generalized modulus of smoothness”, Anal. Math., 25:3 (1999), 187–203
Potapov M. K., Berisha F. M., “On Jackson's theorem for the modulus of smoothness determined by a nonsymmetric generalized shift operator”, Vestnik Moskov. Univ. Ser. I Mat. Mekh., 3 (2000), 7–15, 77; Engl. transl., Moscow Univ. Math. Bull., 55 (2000)
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M. Beriša, F. Berisha, M. K. Potapov, “Polynomial approximation in an integral metric with Jacobi weight”, Vestnik Moskov. Univ. Ser. 1. Mat. Mekh., 1990, no. 5, 33–38