What is the divergence operator $\nabla$ in retarded Bondi coordinates in Minkowski spacetime?
$(t,r, \theta, \phi) \rightarrow (u, r, \theta, \phi)$ with $u=t-r$
The line element is $ds^2 = g^M_{\mu \nu} dx^{\mu}dx^{\nu} = dt^2 -dr^2 -r^2 (d\theta^2 +sin^2\theta d\varphi^2)$