Abstract
Some fast algorithms for computing the eigenvalues of a (block) companion matrix have recently appeared in the literature. In this paper we generalize the approach to encompass unitary plus low rank matrices of the form \(A=U + XY^H\) where U is a general unitary matrix. Three important cases for applications are U unitary diagonal, U unitary block Hessenberg and U unitary in block CMV form. Our extension exploits the properties of a larger matrix \(\hat{A}\) obtained by a certain embedding of the Hessenberg reduction of A suitable to maintain its structural properties. We show that \(\hat{A}\) can be factored as product of lower and upper unitary Hessenberg matrices possibly perturbed in the first k rows, and, moreover, such a data-sparse representation is well suited for the design of fast eigensolvers based on the QR iteration. The resulting eigenvalue algorithm is fast and backward stable.
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The research of the last two authors was partially supported by GNCS project “Analisi di matrici sparse e data-sparse: metodi numerici ed applicazioni”and by the project sponsored by University of Pisa under the Grant PRA-2017-05.
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Bevilacqua, R., Del Corso, G.M. & Gemignani, L. Fast QR iterations for unitary plus low rank matrices. Numer. Math. 144, 23–53 (2020). https://doi.org/10.1007/s00211-019-01080-4
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DOI: https://doi.org/10.1007/s00211-019-01080-4