K. K. Kudaybergenov, “$\nabla$-Fredgolm operators in Banach-Kantorovich spaces”, MFAT, vol. 12, no. 3, pp. 234–242, Sep. 2006, Accessed: Feb. 04, 2026. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/316