The Bi-CG, CGS and Bi-CGSTAB for solving a linear system obtained after the discretization of the two-dimensional Laplacian with an inhomogenous sinusoidal term on the unit square with general boundary conditions are considered. In this case, eigenvalues and eigenvectors of the system can be given theoretically. Using residual polynomials, quantities of rounding errors are separated from residual norms of the Bi-CG, CGS and Bi-CGSTAB. Some new numerical results for estimating and comparing the quantities of the round-off errors of these methods ar given. Moreover, the convergence behaviours in the presence of the rounding errors are observed and analyzed.
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