Accuracy of the Post-Newtonian Approximation for Extreme-Mass Ratio   Inspirals from Black-hole Perturbation Approach
  We revisit the accuracy of the post-Newtonian (PN) approximation and its region of validity for quasi-circular orbits of a point particle in the Kerr spacetime, by using an analytically known highest post-Newtonian order gravitational energy flux and accurate numerical results in the black hole perturbation approach. It is found that regions of validity become larger for higher PN order results although there are several local maximums in regions of validity for relatively low-PN order results. This might imply that higher PN order calculations are also encouraged for comparable-mass binaries. 
Norichika Sago, Ryuichi Fujita, Hiroyuki Nakano
gr-qc