Perturbation theory for the eigenvalues of factorised symmetric matrices

We obtain eigenvalue perturbation results for a factorised Hermitian matrix H = GJG* where J(2) = I and G has full row rank and is perturbed into G delta G, where delta G is small with respect to G. This complements the ear lier results on the easier case of G with full column rank. Applied to squa re factors G our results help to identify the so-called quasidefinite matri ces as a natural class on which the relative perturbation theory for the ei gensolution can be formulated in a way completely analogous to the one alre ady known for positive definite matrices. (C) 2000 Elsevier Science Inc. Al l rights reserved.