Generalized ghost-free quadratic curvature gravity
  In this paper we study the most general covariant action of gravity up to
terms that are quadratic in curvature. In particular this includes non-local,
infinite derivative theories of gravity which are ghost-free and exhibit
asymptotic freedom in the ultraviolet. We provide a detailed algorithm for
deriving the equations of motion for such actions containing an arbitrary
number of the covariant D'Alembertian operators, and this is our main result.
We also perform a number of tests on the field equations we derive, including
checking the Bianchi identities and the weak-field limit. Lastly, we consider
the special subclass of ghost and asymptotically free theories of gravity by
way of an example.
