Stability of Schwarzschild-like solutions in f(R,G) gravity models
  We study linear metric perturbations around a spherically symmetric static
spacetime for general f(R,G) theories, where R is the Ricci scalar and G is the
Gauss-Bonnet term. We find that unless the determinant of the Hessian of f(R,G)
is zero, even-type perturbations have a ghost for any multi-pole mode. In order
for these theories to be plausible alternatives to General Relativity, the
theory should satisfy the condition that the ghost is massive enough to
effectively decouple from the other fields. We study the requirement on the
form of f(R,G) which satisfies this condition. We also classify the number of
propagating modes both for the odd-type and the even-type perturbations and
derive the propagation speeds for each mode.
