We describe algorithms for computing eigenpairs (eigenvalueeigenvector pairs) of a complex n × n matrix A. These algorithms are numerically stable, strongly accurate, and theoretically efficient (i.e., polynomialtime). We do not believe they outperform in practice the algorithms currently used for this computational problem. The merit of our paper is to give a positive answer to a long-standing open problem in numerical linear algebra.
So the problem of devising an algorithm [for the eigenvalue problem] that is numerically stable and globally (and quickly!) convergent remains open.Our initial quotation followed these words in Demmel's text. It asked for an algorithm which will be numerically stable and for which, convergence, and if possible small complexity bounds, can be established. Today, 17 years after Demmel's text, this demand retains all of its urgency: it is not known if any of the standard numerical linear algebra algorithms satisfies the properties above. For example:• The unshifted QR algorithm terminates with probability 1 but is probably infinite average cost if approximations to the eigenvectors are to be output (see [29]).• The QR algorithm with Rayleigh Quotient shift fails for open sets of real input matrices (see [6,7]).