Postprocessing and Accuracy Checking

    Once the eigenvalues and eigenvectors have been extracted from the postprocessing routine dseupd, the user may check the accuracy of the result by computing the direct residuals or for standard or generalized eigenvalue problems, respectively. In order to compute the above quantities, the matrix vector product routines for and must be supplied, even if they are not used in the reverse communication loop. Residual checking is provided in all drivers.