Grigorieva, S. V., and V. N. Ushakov. “Use Finite Family of Multivalued Maps for Constructing Stable Absorption Operator”. Topological Methods in Nonlinear Analysis, vol. 15, no. 1, Mar. 2000, pp. 75-89, https://apcz.umk.pl/TMNA/article/view/TMNA.2000.006.