Coarse-grid correction ($u_h\leftarrow u_h+P{A}_H^{-1}R(b_h-A_h{u}_h)$)

For residual $r_h=b_h-A_h{u}_h$, restrict to a coarse grid, solve $A_H{e}_H=R{r}_h$ approximately, and update $u_h$ by $P{e}_H$. This corrects error that is smooth on the fine grid but representable on the coarse grid.