SO(3) massive gravity
  In this paper, we propose a massive gravity theory with 5 degrees of freedom.
The mass term is constructed by 3 Stuckelberg scalar fields, which respects
SO(3) symmetry in the fields' configuration. By the analysis on the linear
cosmological perturbations, we found that such 5 d.o.f are free from ghost
instability, gradiant instability, and tachyonic instability.
