Slyusarchuk, V. Yu. “Almost Periodic and Poisson Stable Solutions of Difference Equations in Metric Spaces”. Ukrains’kyi Matematychnyi Zhurnal, vol. 67, no. 12, Dec. 2015, pp. 1707-14, https://umj.imath.kiev.ua/index.php/umj/article/view/2103.