Laiyi Zhu, Xiaojie Zhu & Xing Liu. (1970). Weighted Approximation of $r$-Monotone Functions on the Real Line by Bernstein Operators.
            <i>Analysis in Theory and Applications</i>. <i>27</i> (3).
            239-250.
         doi:10.1007/s10496-011-0239-8