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The estimates of diagonally dominant degree and eigenvalues distributions for the Schur complements of matrices
Journal of Inequalities and Applications volume 2013, Article number: 431 (2013)
Abstract
By applying the properties of Schur complement and some inequality techniques, some new estimates of diagonally dominant degree on the Schur complement of matrices are obtained, which improve the main results of Liu (SIAM J. Matrix Anal. Appl. 27:665-674, 2005) and Liu (Linear Algebra Appl. 432:1090-1104, 2010). As an application, we present some new distribution theorems for eigenvalues of the Schur complement. Finally, we give a numerical example to illustrate the theory results.
MSC:15A45, 15A48.
1 Introduction
Let denote the set of all complex matrices, and (). Denote
We call A a strictly diagonally dominant matrix (abbreviated ) if
A is called an Ostrowski matrix (abbreviated ) (see [1]) if
As in [2], for all and , we call , and the i th dominant degree, α-dominant degree and product α-dominant degree of A, respectively.
For , denote by the cardinality of β and . If , then is the submatrix of A lying in the rows indexed by β and the columns indicated by γ. In particular, is abbreviated to . If is nonsingular, then
is called the Schur complement of A with respect to .
The comparison matrix of A, is defined by
A matrix A is called an M-matrix if there exist a nonnegative matrix B and a real number such that , where is the spectral radius of B. It is well known that A is an H-matrix if and only if is an M-matrix, and if A is an M-matrix, then the Schur complement of A is also an M-matrix and (see [3]). We denote by and the sets of H-matrices and M-matrices, respectively.
The Schur complement has been proved to be a useful tool in many fields such as control theory, statistics and computational mathematics, and many works have been done on it (see [4–8]). Meanwhile, studying the locations of eigenvalues of the Schur complement of matrices is of great significance as shown in [2, 3, 9–14]. In this paper, we present some new estimates of diagonally dominant degree on the Schur complement of matrices and use them to study the distributions for the eigenvalues of the Schur complement of matrices.
The paper is organized as follows. In Section 2, we give several new estimates of the diagonally dominant degree, the α-diagonally dominant degree and product α-diagonally dominant degree on the Schur complement of matrices. In Section 3, several new distribution theorems for eigenvalues of the Schur complements are obtained. In Section 4, we present a numerical example to illustrate the theory results.
2 Estimates of diagonally dominant degree for the Schur complement
In this section, we present several new estimates for the diagonally dominant degree, the α-diagonally dominant degree and product α-diagonally dominant degree of the Schur complement of matrices.
Lemma 1 [5]
If , then .
Lemma 2 [5]
If A is an or A is an , then , i.e., .
Lemma 3 [3]
If A is an or A is an and , then the Schur complement of A is an or an , where is the complement of β in N, and is the cardinality of .
Lemma 4 [2]
Let , , and . Then
Theorem 1 Let , , , , and let . Then for all ,
and
where
Proof Since , then , . From Lemma 1 and Lemma 2, we have
Thus, for ,
Similarly to the proof of Lemma 4 in [13], we can prove that . Thus, we obtain Inequation (1). Similarly, we can prove Inequation (2). □
Remark 1 Note that
This shows that Theorem 1 improves Theorem 1 of [13].
Theorem 2 Let , , , , and let . Then for all , ,
and
where
Proof Since , then , . From Lemma 1 and Lemma 2, we have
Thus, for all , ,
Let
By the proof of Theorem 1, we have
Similarly,
By Lemma 4, we have
Thus, we obtain Inequation (3). Similarly, we can prove Inequation (4). □
Remark 2 Note that
This shows that Theorem 2 improves Theorem 2 of [2].
Similarly to the proof of Theorem 2, we can prove the following theorem.
Theorem 3 Let , , , and . Then for all , ,
and
3 Distribution for eigenvalues of the Schur complement
In this section, we present two new distribution theorems for eigenvalues of the Schur complement.
Lemma 5 [2]
Let and . Then for any eigenvalue λ of A, there exists such that
Theorem 4 Let , , , and let . Then for any eigenvalue λ of , there exists such that
Proof Let λ be an eigenvalue of . From the famous Gerschgorin circle theorem, we know that there exists such that . Hence
that is
Thus, Inequation (5) holds. □
Theorem 5 Let , , , , and let . Then for any and every eigenvalue λ of , there exists such that
Proof Let λ be an eigenvalue of . By Lemma 5, we know that there exists such that
Thus,
Similarly to the proof of Theorem 2, we can prove
Therefore, we have
That is, Inequation (6) holds. □
4 A numerical example
In this section, we present a numerical example to illustrate the theory results.
Example 1 Let
By calculation with Matlab 7.1, we have that
Since , by Theorem 4, any eigenvalue λ of satisfies
From Theorem 3 of [2], any eigenvalue λ of satisfies
Evidently, , we use Figure 1 to show the fact.
Meanwhile, since , by taking in Theorem 5, any eigenvalue λ of satisfies
From Theorem 4 of [2], any eigenvalue λ of satisfies
Evidently, , we use Figure 2 to show the fact.
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Acknowledgements
The authors are very indebted to the referees for their valuable comments and corrections, which improved the original manuscript of this paper. This work was supported by the National Natural Science Foundation of China (71161020, 11361074) and IRTSTYN.
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Li, Yt., Wang, F. The estimates of diagonally dominant degree and eigenvalues distributions for the Schur complements of matrices. J Inequal Appl 2013, 431 (2013). https://doi.org/10.1186/1029-242X-2013-431
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DOI: https://doi.org/10.1186/1029-242X-2013-431