Y. J. Piao & H. Nan. (1970). Common Fixed Points for a Countable Family of Quasi-Contractive Mappings on a Cone Metric Space with the Convex Structure.
            <i>Analysis in Theory and Applications</i>. <i>29</i> (3).
            255-266.
         doi:10.4208/ata.2013.v29.n3.5