A family of degenerate elliptic operators: maximum principle and its   consequences
  In this paper we investigate the validity and the consequences of the maximum principle for degenerate elliptic operators whose higher order term is the sum of "k" eigenvalues of the Hessian. In particular we shed some light on some very unusual phenomena due to the degeneracy of the operator. We prove moreover Lipschitz regularity results and boundary estimates for problems in convex domains which are intersections of ball of same radius. We prove also that smooth bounded strictly convex domains are part of this class. As a consequence we obtain the existence of solutions of the Dirichlet problem and of principal eigenfunctions. 
Isabeau Birindelli, Giulio Galise, Hitoshi Ishii
math.AP math.SP