The operator

may be written as

using x$4=$ ict. This is the $4-$D Laplacian, sometimes called the d$$Alembertian and denoted by $\backslash$geoquad $2.$ Show that it is a scalar operator, that is$,$ invariant under Lorentz transformations, i$.$e$.,$ under rotations in the space of vectors $($x$1,$ x$2,$ x$3,$ x$4).$