Gonska, H., and R. Păltănea. “Quantitative Convergence Theorems for a Class of Bernstein–Durrmeyer Operators Preserving Linear Functions”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 62, no. 7, July 2010, pp. 913–922,
https://umj.imath.kiev.ua/index.php/umj/article/view/2924.