Zapryanova, Teodora Dimova, Souroujon, Diko Mois .
Detailed Study on Characterization of Best Algebraic Approximation by a Generalized Modulus of Smoothness.
Hooghly, India: B P International, 2021, pp. 159 - 169, ISBN 978-93-90768-00-4 (print) ; 978-93-90768-06-6.