In this paper, we show that PML solutions are most readily derived from the well-understood 1-D Robin boundary value classically defined as a weighted combination of Dirichlet and Neumann constituents. The derivation, thus, uses no split field or stretched coordinate reformulation. Continuing with this classical construction, in fact, gives the complete superposition solution valid for arbitrary boundary orientations and operators. This further enables the explanation that uniaxial PML matrix solves only the coordinately aligned terminations. Temporal implicit and spatial explicit representations are presented. Solving Robin impedance-matched solutions, thus, amounts to merely relabeling the corresponding boundary variable name. Following the superposition principle, we demonstrate mathematically, geometrically, and numerically that the PML reflection-free boundary is also composed of one Dirichlet and one Neumann acting out-of-phase solutions. Relating this to reflectional symmetry is presented.
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