Volpert, Vitaly, and Vitali Vougalter. “Solvability Conditions For Some Non Fredholm Operators in a Layer in Four Dimensions”. Discontinuity, Nonlinearity, and Complexity 3, no. 1 (March 1, 2014): 59–71. Accessed October 17, 2026. https://lhscientificpublishing.com/index.php/DNC/article/view/2052.