Kalita, E. A. “Morrey Regularity of Solutions of Nonlinear Elliptic Systems of Arbitrary Order With Restrictions on the Modulus of Ellipticity”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 54, no. 11, Nov. 2002, pp. 1452-66,
https://umj.imath.kiev.ua/index.php/umj/article/view/4184.