Baernstein II, Albert, and Leonid V. Kovalev. 2009. “On Hölder Regularity for Elliptic Equations of Non-Divergence Type in the Plane”. ANNALI SCUOLA NORMALE SUPERIORE - CLASSE DI SCIENZE 4 (2):295-317. https://doi.org/10.2422/2036-2145.2005.2.04.