GorodniiM. F., and DorogovtsevA. Y. “Bounded Solutions of One Class of Nonlinear Operator Difference Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 47, no. 7, July 1995, pp. 890–896, https://umj.imath.kiev.ua/index.php/umj/article/view/5483.