Solving this equation using the method of spherical means yields Kirchhoff’s formula in R3. It’s not the Schrödinger (10[1])equation, but applying the (10[1])WKB approximation to this equation yields the eikonal (“icon-al”) equation. (10[2])Linearizing the metric in the (10[1])Einstein field equations and transforming to the harmonic gauge gives an instance of this equation, (10[1])as does using the curl-of-curl identity (10[1])on two of Maxwell’s (10[1])equations. Any function of the form f of (10[1])quantity x plus-or-minus vt (10[1])solves this equation, (10[1])which can be written as “the d’Alembertian (10[1])(“dal-ahm-BARE-shin”) of u equals zero.” (10[2])This second-order PDE (-5[2])can be derived by treating a (10[1])1D string as a continuum (-5[1])of springs, (10[1]-5[1])yielding solutions that travel with speed (10[1])“square-root of T over mu.” For 10 points, name this generically-named equation (10[1])that describes disturbances that propagate through (10[1])media. ■END■ (10[11]0[6])