Solving this equation using the method of spherical means yields Kirchhoff’s formula in R3. It’s not the Schrödinger equation, but applying the WKB approximation to this equation yields the eikonal (“icon-al”) equation. Linearizing the metric in the Einstein field equations and transforming to the harmonic gauge gives an instance of this equation, as does using the curl-of-curl identity on two of Maxwell’s equations. Any function of the form f of quantity x plus-or-minus vt (10[1])solves this equation, which can be written as “the d’Alembertian (10[1])(“dal-ahm-BARE-shin”) of u equals zero.” This second-order PDE (-5[1])can be derived by treating a 1D string as a continuum of springs, yielding solutions that travel with speed “square-root of T over mu.” For 10 points, name this generically-named equation that describes disturbances that propagate through (10[1])media. ■END■