Gefter, S. L., and A. L. Piven’. “Entire Solutions of One Linear Implicit Differential-Difference Equation in Banach Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 70, no. 8, Aug. 2018, pp. 1044-57, https://umj.imath.kiev.ua/index.php/umj/article/view/1616.