ZhuravlevV. F. “Bifurcation Conditions for the Solutions of Weakly Perturbed Boundary-value problems for Operator Equations in Banach Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 70, no. 3, Mar. 2018, pp. 366-78, https://umj.imath.kiev.ua/index.php/umj/article/view/1562.