Abstract. We present a chart of structured backward errors for approximate eigenpairs of singly and doubly structured eigenvalue problems. We aim to give, wherever possible, formulae that are inexpensive to compute so that they can be used routinely in practice. We identify a number of problems for which the structured backward error is within a factor √ 2 of the unstructured backward error. This paper collects, unifies, and extends existing work on this subject.Key words. eigenvalue, eigenvector, symmetric matrix, Hermitian matrix, skew-symmetric matrix, skew-Hermitian matrix, symplectic matrix, conjugate symplectic matrix, Hamiltonian matrix, backward error, condition number AMS subject classifications. 65F15, 65F20, 65H10, 65L15, 65L20, 15A18, 15A57 PII. S089547980139995X1. Introduction. Byers, and Mehrmann [8] present a chart of numerical methods for structured eigenvalue problems for which the matrices have more than one of the properties defined as follows:
−InIn 0 ], I n being the n×n identity matrix. Structured eigenvalue problems occur in numerous applications and we refer to [8] for a list of them and pointers to the relevant literature. In this paper we present a chart of computable backward errors for approximate eigenpairs and condition numbers for simple eigenvalues of matrices having one or two of these special structures.The importance of condition numbers for characterizing the sensitivity of solutions to problems and backward errors for assessing the stability and quality of numerical algorithms is widely appreciated. A backward error of an approximate eigenpair (x, λ) of a matrix A is a measure of the smallest perturbation E such that (A + E)x = λx. This backward error has two main uses. First, it can be used to determine if A natural definition of the normwise backward error of an approximate eigenpairwhere α is a positive parameter that allows freedom in how the perturbations are measured and · denotes any vector norm and the corresponding subordinate matrix norm. Deif [9] derived the explicit expression for the 2-norm (also valid for any subordinate norm and the Frobenius norm),showing that the normwise backward error is a scaled residual. Also of interest is the backward error of a set of approximate eigenpairs (For a measure of the backward error we use the natural generalization of (1.1),for which an explicit expression is available for any unitarily invariant norm if rank(The measure η is not entirely appropriate for our structured eigenvalue problems, as it does not respect any structure in A. Similar remarks can be made about condition numbers. Standard condition numbers are derived without requiring that perturbations preserve structure. As a consequence, standard condition numbers usually exceed the actual condition number for an eigenvalue problem subject to structured perturbation. In the last few years, efforts have been concentrated on deriving new structure-preserving algorithms for the solution of structured eigenvalue problems for both the dense case [21], to cite just a ...