A wave equation for light is

where \(\phi\) is a scalar potential. Show that the set of all linear transformations of the spacetime coordinates that permit this wave equation to be written as we did, correspond to (i) four possible translations in space and time, (ii) three constant rotations of space, and (iii) three Lorentz transformations. Collectively, these are called the Poincaré transformations of spacetime.

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10 + x2 2 + = 0.