Wiśnicki, A. “Hölder Continuous Retractions and Amenable Semigroups of Uniformly Lipschitzian Mappings in Hilbert Spaces”. Topological Methods in Nonlinear Analysis, vol. 43, no. 1, Apr. 2016, pp. 89-96, https://apcz.umk.pl/TMNA/article/view/TMNA.2014.006.