Eigenvalues of the real generalized eigenvalue equation perturbed by a low-rank perturbation |
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Authors: | Tomislav P ?ivkovi? |
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Institution: | (1) Rudjer Bo kovi Institute, 41001 Zagreb, The Republic of Croatia |
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Abstract: | The low-rank perturbation (LRP) method solves the perturbed eigenvalue equation (B +V)
k
=
k
(C +P)
k
, where the eigenvalues and the eigenstates of the related unperturbed eigenvalue equationB
i
=
i
C
i
are known. The method is designed for arbitraryn-by-n matricesB, V, C, andP, with the only restriction that the eigenstates
i
of the unperturbed equation should form a complete set. We consider here a real LRP problem where all matrices are Hermitian, and where in addition matricesC and (C +P) are positive definite. These conditions guarantee reality of the eigenvalues
k
and
i
. In the original formulation of the LRP method, each eigenvalue
k
is obtained iteratively, starting from some approximate eigenvalue
k
. If this approximate eigenvalue is not well chosen, the iteration may sometimes diverge. It is shown that in the case of a real LRP problem, this danger can be completely eliminated. If the rank of the generalized perturbation {V, P} is small with respect ton, then one can easily bracket and hence locate to any desirable accuracy the eigenvalues
k
(k = 1, ...,n) of the perturbed equation. The calculation of alln eigenvalues requiresO( 2
n
2) operations. In addition, if the perturbation (V, P) is local with the localizabilityl p, then onlyO( 2
n) operations are required for a derivation of a single eigenvalue. |
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Keywords: | |
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