Abbasоv A. “Fredholm Quasi-Linear Manifolds and Degree of Fredholm Quasi-Linear Mapping Between Them”. Ukrains’kyi Matematychnyi Zhurnal, vol. 63, no. 5, May 2011, pp. 579-95, https://umj.imath.kiev.ua/index.php/umj/article/view/2744.