Chandok, S., and T. D. Narang. “Common Fixed Points and Invariant Approximation of $R$-Subweakly Commuting Maps in Convex Metric Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 62, no. 10, Oct. 2010, pp. 1367–1376, https://umj.imath.kiev.ua/index.php/umj/article/view/2961.