Coarse-grid correction
= Coarse-grid correction
{title2=$u_h\leftarrow u_h+PA_H^{-1}R(b_h-A_hu_h)$}
For residual $r_h=b_h-A_hu_h$, restrict to a coarse grid, solve $A_He_H=Rr_h$ approximately, and update $u_h$ by $Pe_H$. This corrects error that is smooth on the fine grid but representable on the coarse grid.