Regularity for radial solutions of degenerate fully nonlinear equations
  In this paper we prove Holder regularity of the derivative of radial solutions to fully nonlinear equations when the operator is hessian, homogenous of degree 1 in the Hessian, homogenous of some degree $\alpha>-1$ in the gradient and which is elliptic when the gradient is not null. 
I. Birindelli, F. Demengel
math.AP