Zapryanova, Teodora Dimova A Characterization of the K-functional for the Algebraic Version of the Trigonometric Jackson Integrals Gs,n and the K-functionals for Cao-Gonska Operators Gs,n* and Gs,n+,
Birkhauser, 2009, pp. 397 - 413, ISSN 1422-6383 (print) ; 1420-9012 (online).