Zinchenko, T. N., and A. A. Murach. “Douglis-Nirenberg Elliptic Systems in Hörmander Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 64, no. 11, Nov. 2012, pp. 1477-6, https://umj.imath.kiev.ua/index.php/umj/article/view/2674.