PolulyakhE. O., and VlasenkoI. Y. “On Iteration Stability of the Birkhoff Center With Respect to Power 2”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 58, no. 5, May 2006, pp. 705–707, https://umj.imath.kiev.ua/index.php/umj/article/view/3487.