= Solution
The <retarded and advanced null coordinates> invert to
$$
t=\frac{u+v}{2},\qquad z=\frac{v-u}{2}.
$$
Thus
$$
-dt^2+dz^2=-du\,dv
$$
and the <Minkowski metric> becomes
$$
\boxed{ds^2=-du\,dv+dx^2+dy^2.}
$$
The <coordinate basis> transforms by the chain rule:
$$
\boxed{
\partial_t=\partial_u+\partial_v,\qquad
\partial_z=-\partial_u+\partial_v,
}
$$
or equivalently
$$
\boxed{
\partial_u=\frac12(\partial_t-\partial_z),\qquad
\partial_v=\frac12(\partial_t+\partial_z).
}
$$
In the $(z,t)$ diagram, $\partial_t$ points vertically upward and $\partial_z$ horizontally right. The vector $\partial_u$ points along the future-left null ray and $\partial_v$ along the future-right null ray; their factors of one half affect length in the coordinate drawing but not direction.
Using these derivative relations, the <Minkowski wave operator in null coordinates> is
$$
\boxed{
\Box=-\partial_t^2+\partial_z^2+\partial_x^2+\partial_y^2
=-4\partial_u\partial_v+\partial_x^2+\partial_y^2.
}
$$
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