Baernstein II, A., and L. V. Kovalev. “On Hölder Regularity for Elliptic Equations of Non-Divergence Type in the Plane”. ANNALI SCUOLA NORMALE SUPERIORE - CLASSE DI SCIENZE, vol. 4, no. 2, Dec. 2009, pp. 295-17, doi:10.2422/2036-2145.2005.2.04.