the first step is to neglect and to extend the discrete operator to an infinite grid; the second step assumes that any discrete nonlinear operator can be linearized locally (by freezing coefficients) and can be replaced locally by an operator with constant coefficients [47]. This method has been successfully applied to obtain a bet- ter understanding of MG algorithms applied to nonlinear problems or linear problems with discontinuous coefficients [1, 34, 15, 10, 4].