1. A.D. Alexandrov, Uniqueness condition and estimates of the solution of Dirichlet's problem, Vestmik Leningrad 13 (1963), no. 3, 5–39.
2. I. Bakelman, Theory of quasilinear elliptic equations, Siberian Math J. 2 (1961), 179–186.
3. R. Bass, The Dirichlet problem for radially homogeneous elliptic operators, Trans. AMS 320 (1990), 593–674.
4. R. Bass and K. Burdzy, The boundary Harnack principle for non-divergence form elliptic operators, preprint.
5. B. Barcelo, L. Escauriaza and E. Fabes, Gradient estimates at the boundary for solutions to nondivergence elliptic equations, Contemp. Math. 107 (1990), 1–12.