Compact spacelike surfaces whose mean curvature function satisfies a   nonlinear inequality in a 3-dimensional Generalized Robertson-Walker   spacetime
  Spacelike surfaces in Generalized Robertson-Walker spacetimes whose mean curvature function satisfies a natural nonlinear inequality are analyzed. Several uniqueness and nonexistence results for such compact spacelike surfaces are proved. In the nonparametric case, new Calabi-Bernstein type problems are solved as a consequence. 
Alfonso Romero and Rafael M. Rubio
math.DG