NizhnikL. P., and TarasovV. G. 1999. “Scattering Problem for a Multidimensional System of First-Order Partial Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal 51 (8), 1065–1076. https://umj.imath.kiev.ua/index.php/umj/article/view/4697.