Kudaybergenov, K. K. “$\nabla$-Fredgolm Operators in Banach-Kantorovich Spaces”. Methods of Functional Analysis and Topology 12, no. 3 (September 25, 2006): 234–242. Accessed December 16, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/316.