Illustration of the prolongation of the coarse-grid correction to the fine grid using data from the ghost layer. Solid black line: the boundary layer between the domains and . Dashed line: the ghost layer of the domain . Dotted line and the empty circle: the fine-grid layer near to the domain boundary and a fine-grid node. Gray lines: lines of the coarse grid. The arrows show how the data are interpolated to the fine-grid node using the cubic-centered interpolation of the coarse-grid nodes data.