On the coarsest grid, an effective unilevel solver is used for accurate solution; here, we shall use the semi-implicit method represented by Equation (19). Without reducing the importance of the restriction and interpolation operators, the efficiency of every MG method strongly relies on the efficiency of the relaxation method, also known as smoother, used at each level in reducing
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International Journal of Computer Mathematics 151
(or smoothing) the high-frequency components of the error.