Xia Pan, Zuohuan Zheng & Zhe Zhou. (1970). Ergodic Behaviour of Nonconventional Ergodic Averages for Commuting Transformations.
            <i>Journal of Nonlinear Modeling and Analysis</i>. <i>1</i> (4).
            513-525.
         doi:10.12150/jnma.2019.513