Consider a regular rectangular 2D $r$-$z$ mesh with constant spacing $\Delta r$ along the $r$-axis with $n$ points and spacing $\Delta z$ along the $z$-axis with $m$ points. I thought it would be a straightforward formula for node connectivity/node numbering. But searching the internet is finding complicated ideas for general situations.
What is the formula for the node number of the point $(i, j)$ in a regular rectangular 2D $r$-$z$ mesh with constant spacing?