A Cost-Effective Multigrid Projection Operator (to appear in Journal of Computational and Applied Mathematics.) Jun Zhang Department of Mathematics, The George Washington University Washington, DC 20052, USA zhang@math.gwu.edu http://gwis2.circ.gwu.edu/~yaohan February 1, 1996, revised April 20, 1996 Abstract A novel heuristic residual analysis is proposed to derive a computationally cost-effective residual projection operator in multigrid with the five-point Red-Black Gauss-Seidel relaxation for solving the two dimensional Poisson equation. This optimal residual injection operator is as cheap as the trivial injection operator, but is more efficient than the costly full-weighting operator and achieves near-optimal convergence rate. 1991 Mathematical Subject Classification: 65F10, 65N06, 65N22, 65N55. Key words and phrases: Multigrid method, residual projection, Poisson equation, Red-Black Gauss-Seidel.