Prove the invariance of the electromagnetic wave equation in relativity by showing that the corresponding differential operator is an invariant. That is, show that
∂²/∂x² + ∂²/∂y² + ∂²/∂z² - (1/c²) ∂²/∂t²
= ∂²/∂x'² + ∂²/∂y'² + ∂²/∂z'² - (1/c²) ∂²/∂t'²
when the space-time variables are related by the Lorentz transformations (see Problem 1-8).