Kovtonyuk, D. A., and R. R. Salimov. “Asymptotic Behavior of Generalized Quasiisometries at a Point”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 63, no. 4, Apr. 2011, pp. 481-8, https://umj.imath.kiev.ua/index.php/umj/article/view/2734.