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method of descent

Recovering a lower-dimensional wave-equation solution formula by treating its data as a special, direction-independent case of the higher-dimensional formula.

The method of descent derives the 2D (and then 1D) wave equation's solution formula by substituting data that doesn't depend on one of the spatial coordinates into Kirchhoff's 3D formula -- the source of the fact that 2D waves, unlike 3D ones, leave a persistent wake (Huygens' principle).

See also

Kirchhoff's formula Huygens' principle
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