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…the medium.
If the charge density is zero all over the region, the Poison's
Equation becomes Laplace's equation:
Wu, D. and Chen, J., “Efficient characterizations of composite materials electrical properties based on GPU accelerated finite difference method,”
in IEEE Antennas and Propagation Society International Symposium, Toronto, Canada, 2010.
32…
https://www.wichita.edu/academics/engineering/ECE/research/CAPPLab/_documents/PDF/DRZ_Recent_Work_2.pdf