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## Accuracy Issues

As usual in numerical linear algebra, we will use the backward error analysis model to assess the accuracy of the results. We will first give perturbation theorems that tell how much a perturbation of the data, in this case matrix elements, will change the results, eigenvalues, singular values and vectors. We will also provide a posteriori methods to measure the backward error for computed solutions.

In the simplest cases, the perturbation theorems give a bound for the perturbation of the eigenvalues as a multiple of the perturbation of the matrix elements. The multiplier is called a condition number. For eigenvectors we also need information about the distance between the different eigenvalues, as well as the angles between left and right eigenvectors. In degenerate cases, the best we can get is an asymptotic series, possibly involving fractional powers of the perturbation.

We will discuss what to do when a simple condition number based bound is not practical. If we do not have good bounds for individual eigenvectors, then a better conditioned invariant (or deflating or reducing) subspace of higher dimension may be available. We can also derive a bound for the norm of the resolvent and find pseudospectra.

For iterative algorithms a posteriori methods are used to compute the backward error, often using computed residuals. It is possible to compute, or at least estimate, the residual during the computation as part of monitoring for convergence.

We will show when stronger results as e.g. small relative error bounds for small eigenvalues exist. This is of importance, specially when an ill conditioned matrix comes from the discretization of a well conditioned continuous problem.

Jack Dongarra
Wed Jun 21 02:35:11 EDT 1995