Some remarks on singular solutions of nonlinear elliptic equations. III:   viscosity solutions, including parabolic operators
  The paper concerns singular solutions of nonlinear elliptic equations, which include removable singularities for viscosity solutions, a strengthening of the Hopf Lemma including parabolic equations, Strong maximum principle and Hopf Lemma for viscosity solutions including also parabolic equations. 
Luis Caffarelli, YanYan Li and Louis Nirenberg
math.AP math.DG