On the Perturbation Theory for Unitary Eigenvalue Problems
B. Bohnhorst, Angelika Bunse‐Gerstner, Heike Faßbender · SIAM Journal on Matrix Analysis and Applications · 2000
Some aspects of the perturbation theory for eigenvalues of unitary matrices are considered. Making use of the close relation between unitary and Hermitian eigenvalue problems a Courant--Fischer-type theorem for unitary matrices is derived and an inclusion theorem analogous to the Kahan theorem for Hermitian matrices is presented. Implications for the special case of unitary Hessenberg matrices are discussed.