Dudkin, M.E., and O. Yu. Dyuzhenkova. “ Singularly Perturbed Rank One Linear Operators”. Matematychni Studii 56, no. 2 (December 26, 2021): 162-175. Accessed June 25, 2022. http://matstud.org.ua/ojs/index.php/matstud/article/view/209.