Parabolic systems of nonlinear second order equations; elliptic systems of nonlinear second order equations; elliptic equations in nonsmooth domains, especially with conical and angular points on the boundary.
D. V. Portnyagin, “The maximum and comparison principles for a system of two equations”, Sibirsk. Mat. Zh., 53:2 (2012), 365–376; Siberian Math. J., 53:2 (2012), 291–300
2009
2.
D. V. Portnyagin, “Schauder Estimates and Complete Regularity of the Solutions of an Elliptic System”, Mat. Zametki, 85:1 (2009), 89–97; Math. Notes, 85:1 (2009), 82–89