Faierman, M. “Fredholm Theory Connected With a Douglis-Nirenberg System of Differential Equations over $\mathbb{R}^n$”. Methods of Functional Analysis and Topology 22, no. 4 (December 25, 2016): 330–345. Accessed December 16, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/645.