A Generalized Maximum Principle for Yau's Square Operator, with   Applications to the Steady State Space
  We derive, for the square operator of Yau, an analogue of the Omori-Yau maximum principle for the Laplacian. We then apply it to obtain nonexistence results concerning complete spacelike hypersurfaces with constant higher order mean curvature in the Steady State space. 
Antonio Caminha and Henrique de Lima