My friend was working on this, and he asked me about the stability of PML while applying on pseudo-spectral method, I believe his concern was how to choose the difference(if the difference should be chosen with a directional preference, forward/backward, or inflow/outflow) such that the numerical solution is stable over time. I am nowhere an expert on this topic and do not even know how to choose the keywords to google, so here I am requesting some references. Thanks a lot.
How to choose a stable PML for pseudo-spectral method with strongly varying velocity
finite difference;reference request