Before revision:  The modified-RFB is able to solve the Helmholtz equation efficiently in 2D up to ch = 3.5 where c is the wave number and h is the mesh size  .
After revision:  The modified-RFB is able to solve the Helmholtz equation efficiently in 2D up to ch = 3.5 where c is the wave number and h is the mesh size . We provide analysis to show how the AB method mitigates the pollution error .