The system on the coarsest level must be solved exactly. If the system is small enough, as in the example of the multigrid workbench, a single (or, more generally a few) sweeps of the smoother may serve as a solver. Any other method my be chosen as well, as long as the process does not add significantly to the work count.
Ulrich Ruede , Thu Feb 2 21:06:00 MEZ 1995
Updated by Craig C. Douglas