This frequency-domain response problem leads to an  system of linear equations as described above. An early description of a frequency-domain response FDTD method to solve scattering problems was published by Christ and Hartnagel (1987).
The issue of frequency-domain responses results in a system of linear equations, as previously mentioned. A pioneering work on using the FDTD method for solving scattering problems was published by Christ and Hartnagel in 1987.