An $ f(R) $ gravitation for galactic environments
  We propose an action-based $ f(R) $ modification of Einstein's gravity which admits of a modified Schwarzschild-deSitter metric. In the weak field limit this amounts to adding a small logarithmic correction to the newtonian potential. A test star moving in such a spacetime acquires a constant asymptotic speed at large distances. This speed turns out to be proportional to the fourth root of the mass of the central body in compliance with the Tully-Fisher relation. A variance of MOND's gravity emerges as an inevitable consequence of the proposed formalism. It has also been shown (Mendoza et al. 2006) that a) the gravitational waves in this spacetime propagate with the speed of light in vacuum and b) there is a lensing effect additional to what one finds in the classic GR. 
Y. Sobouti