Variations of Infinite Derivative Modified Gravity
  We consider nonlocal modified Einstein gravity without matter, where nonlocal
term has the form $P(R) F(\Box) Q(R)$. For this model, in this paper we give
the derivation of the equations of motion in detail. This is not an easy task
and presented derivation should be useful to a researcher who wants to
investigate nonlocal gravity. Also, we present the second variation of the
related Einstein-Hilbert modified action and basics of gravity perturbations.
