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2. R. Păltănea, “A class of Durrmeyer-type operators preserving linear functions,” Ann. Tiberiu Popoviciu Sem. Funct. Equat. Approxim. Convex. (Cluj-Napoca), 5, 109–117 (2007).
3. H. Gonska, Quantitative Aussagen zur Approximation durch positive lineare Operatoren, Doctoral-Degree Thesis, University of Duisburg (1979).
4. H. Gonska, D. Kacsó, and P. Piţul, “The degree of convergence of overiterated positive linear operators,” J. Appl. Funct. Anal., 1, 403–423 (2006).
5. H. Gonska and I. Raşa, “A Voronovskaya estimate with second-order modulus of smoothness,” Proc. Math. Inequal. (Sibiu / Romania, Sept. 2008) (to appear).