V. Gupta, N. Deo and X. M. Zeng. (2013). Simultaneous Approximation for Szász-Mirakian-Stancu-Durrmeyer Operators.
            <i>Analysis in Theory and Applications</i>. <i>29</i> (1).
            86-96.
         doi:10.4208/ata.2013.v29.n1.9