A PML in FDFD. Because the FDTD method yields a steady-state solution, and it cannot be excited by a Gaussian source, or have sources turned off at convenient times, the PML is even more important than it is in the FDTD method. Derive the stretched-coordinate PML in the FDFD method using the wave equation, using the same sx , etc., stretching parameters as in the UPML. Hint: start from Equations (9.57) and combine them to form a wave equation, then discretize. For more details on the PML in the FDTD method, see the article by Rappaport et al. [2].