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Applied Mathematics E-Notes, 9(2009), 289-296 c ISSN 1607-2510 Available free at mirror sites of http://www.math.nthu.edu.tw/∼amen/ Perron Complements Of Diagonally Dominant Matrices And H-Matrices∗ Li Zeng†, Ming Xiao‡, Ting-Zhu Huang§ Received 27 November 2008 Abstract In this paper, we consider properties of the Perron complements of diagonally dominant matrices and H-matrices. 1 Introduction Let A = (aij ) be an n × n matrix, and recall that A is (row) diagonally dominant if |aii | ≥ |aij |, i = 1, 2, ..., n. (1) j=i A is further said to be strictly diagonally dominant if all the strict inequalities in (1) hold. Obviously the principal submatrices of strictly diagonally matrices are strictly diagonally dominant and thus A is nonsingular. For A ∈ Z = {(aij ) ∈ Rn,n : aij ≤ 0, i = j}, if A = aI − B, B ≥ 0, a > ρ (B) , then A is called an M-matrix. The comparison matrix µ (A) = (µij ) is deﬁned by − |aij | i = j, µij = |aij | i = j. A ∈ C n,n is called an H-matrix if µ (A) is an M-matrix. If there exists a positive diagonal matrix D = diag (d1 , ..., dn) such that D−1 AD is strictly diagonally dominant, we call A a generalized diagonally dominant matrix. It is well-known that A is an H- matrix is equivalent to A is generalized diagonally dominant. The empty set is denoted by φ. Let α, β be nonempty ordered subsets of n := {1, 2, ..., n}, both consisting of strictly increasing integers. By A (α, β) we shall denote the submatrix of A lying in rows indexed by α and columns indexed by β. If, in addition, α = β, then the principal submatrix A (α, α) is abbreviated to A (α). ∗ Mathematics Subject Classiﬁcations: 15A48 † Collegeof Computer Science and Technology, Southwest University for Nationalities, Chengdu, Sichuan, 610041, P. R. China ‡ College of Computer Science and Technology, Southwest University for Nationalities, Chengdu, Sichuan, 610041, P. R. China § School of Applied Mathematics, University of Electronic Science and Technology of China, Chengdu, 610054, P. R. China 289 290 Perron Complements of Matrices Suppose that α ⊂ n . If A (α) is nonsingular, then the Schur complement of A (α) in A is given by −1 S (A/A (α)) = A (β) − A (β, α) [A (α)] A (α, β) , (2) where β = n \α. A well-known result due to Carlson and Markham [1] states that the Schur complements of strictly diagonally dominant matrices are diagonally dominant. For an n × n nonnegative and irreducible matrix A, Meyer [2,3] introduced the notion of the Perron complement. Again, let α ⊂ n and β = n \α. Then the Perron complement of A (α) in A is given by −1 P (A/A (α)) = A (β) + A (β, α) [ρ (A) I − A (α)] A (α, β) , (3) where ρ (·) denotes the spectral radius of a matrix. Recall that as A is irreducible, ρ (A) > ρ (A (α)), so that the expression on the right-hand side of (3) is well deﬁned, and we observe that ρ (A) I − A (α) is an M-matrix and thus (ρ (A) I − A (α))−1 ≥ 0. Meyer [2,3] has derived several interesting and useful properties of P (A/A (α)), such as P (A/A (α)) is also nonnegative and irreducible, and ρ(P (A/A (α))) = ρ (A). In addition, the Perron complements of inverse M-matrices [4] have also been studied. For any α ⊂ n and for any t ≥ ρ (A), let the extended Perron complement at t be the matrix Pt (A/A (α)) = A (β) + A (β, α) [tI − A (α)]−1 A (α, β) , (4) which is also well deﬁned since t ≥ ρ (A) > ρ (A (α)). In this paper, we shall show, in Section 2, that the Perron complement of a diago- nally dominant and nonnegative irreducible matrix , −1 P (A/A (α)) = A (β) + A (β, α) [ρ (A) I − A (α)] A (α, β) , n is diagonally dominant only if ρ (A) ≥ maxi∈α |aij |. In Section 3, we shall show a j=1 similar result for H-matrices. 2 Perron Complements of Diagonally Dominant Ma- trices First recall the following result proved in [2]. LEMMA 2.1 ([2]). If A is a nonnegative irreducible matrix with spectral radius ρ (A), and let α ⊂ n , α = φ and β = n \α. Then the Perron complement P (A/A (α)) = A (β) + A (β, α) [ρ (A) I − A (α)]−1 A (α, β) is also a nonnegative irreducible matrix with spectral radius ρ (A). We are now in a position to state the main result of the paper on the Perron complements of diagonally dominant matrices. Zeng et al. 291 THEOREM 2.2. Let A be an n×n diagonally dominant and nonnegative irreducible matrix with spectral radius ρ (A), and let α ⊂ n , α = φ and β = n \α. Then, for n ρ (A) ≥ maxi∈α |aij |, j=1 P (A/A (α)) = A (β) + A (β, α) [ρ (A) I − A (α)]−1 A (α, β) is a diagonally dominant and nonnegative irreducible matrix. PROOF. Let α = {i1 , i2 , ..., ik} and β = {j1 , j2 , ..., jl}, where k + l = n. Denote |A| = (|aij |). Since A is a diagonally dominant matrix, we have, for any i ∈ n , |aii | ≥ |aij | j=i or l k |ajt jt | ≥ |ajt js | + |ajt is |, (5) s=1,=t s=1 where jt , js ∈ β, is ∈ α. Note that A is an irreducible and nonnegative matrix, then ρ (A) > ρ (A (α)), so that ρ (A) I − A (α) is an M-matrix. Then we have (ρ (A) I − A (α))−1 ≥ 0 and aij ≥ 0. (6) n By ρ (A) ≥ maxi∈α |aij |, we have j=1 l k ρ (A) ≥ maxi∈α |aij | + |aij | = maxiv ∈α aiv jt + maxiv ∈α aiv it . (7) j∈β j∈α t=1 t=1 l If maxiv ∈α aiv jt = 0, then P (A/A (α)) = A (β). Thus, the matrix P (A/A (α)) is t=1 l diagonally dominant. If maxiv ∈α aiv jt > 0, then, by (7), we have t=1 l k 0 < maxiv ∈α aiv jt ≤ ρ (A) − maxiv ∈α ai v i t . (8) t=1 t=1 Thus l ai v j t t=1 maxiv ∈α k ≤ 1. (9) ρ (A) − ai v i t t=1 Denote l l † −1 x = (ρ (A) I − A (α)) ai1 js , ..., ai k j s (10) s=1 s=1 292 Perron Complements of Matrices or l l † (ρ (A) I − A (α)) x = ai1 js , ..., ai k j s . s=1 s=1 Letting xv = max{x1 , x2 , · · · , xk }, where xi is the i-th component of x, we obtain l k ai v j s = (ρ (A) − aiv iv ) xv + (−aiv it )xt s=1 t=1,=v k ≥ (ρ (A) − aiv iv + (−aiv it ))xv t=1,=v k = (ρ (A) − aiv it )xv . t=1 By (8), we have l l ai v j t ai v j t t=1 t=1 xv ≤ k ≤ maxiv ∈α k . ρ (A) − ai v i t ρ (A) − ai v i t t=1 t=1 By (9), we have xv ≤ 1. (11) Denote the (t, s)-entry of P (A/A (α)) by ajt js . Then, for t = 1, 2, ..., l, we have l aj t j t − aj t j s s=1,=t ai 1 j t + (ajt i1 , ..., ajtik ) (ρ (A) I − A (α)) . −1 = aj t j t . . ai k j t l ai 1 j s + (ajt i1 , ..., ajtik ) (ρ (A) I − A (α)) . −1 − aj t j s . . , s=1,=t ai k j s Zeng et al. 293 so that l aj t j t − aj t j s s=1,=t |ai1 jt | −1 . ≥ |ajt jt | − (|ajt i1 | , ..., |ajt ik |) (ρ (A) I − A (α)) . . |aik jt | l |ai1 js | −1 . − |ajt js | + (|ajt i1 | , ..., |ajt ik |) (ρ (A) I − A (α)) . . s=1,=t |aik js | l s=1 ai1 js l ajt js − (ajt i1 , ..., ajtik ) (ρ (A) I − A (α)) . −1 = aj t j t − . . s=1,=t l ai k j s s=1 l 1 ajt js − (ajt i1 , ..., ajtik ) . . ≥ aj t j t − . s=1,=t 1 l k = |ajt jt | − |ajt js | − |ajt is | s=1,=t s=1 ≥ 0. It follows that P (A/A (α)) is a diagonally dominant matrix. By Lemma 2.1, the matrix P (A/A (α)) is nonnegative irreducible. This completes the proof. By Theorem 2.2, we have several immediate results about the extended Perron complements and the Perron complements of strictly diagonally dominant matrices. COROLLARY 2.3. Let A be an n × n diagonally dominant and nonnegative ir- reducible matrix with spectral radius ρ (A), and let α ⊂ n , α = φ and β = n \α. n Then, for any t ∈ [ρ (A) , ∞) and ρ (A) ≥ max |aij |, i∈α j=1 −1 Pt (A/A (α)) = A (β) + A (β, α) [tI − A (α)] A (α, β) is a diagonally dominant and nonnegative irreducible matrix. COROLLARY 2.4. Let A be an n × n strictly diagonally dominant and non- negative irreducible matrix with spectral radius ρ (A), and let α ⊂ n , α = φ and n β = n \α. Then, for any t ∈ [ρ (A) , ∞) and ρ (A) ≥ maxi∈α |aij |, P (A/A (α)) j=1 and Pt (A/A (α)) are strictly diagonally dominant and nonnegative irreducible matri- ces. 294 Perron Complements of Matrices 3 Perron Complements of H-matrices In this section, we obtain a theorem of the Perron complements of H-matrices. THEOREM 3.1. Let A be an n × n nonnegative irreducible H-matrix with spec- tral radius ρ (A), and let α ⊂ n , α = φ and β = n \α. Then, for ρ (A) ≥ n maxi∈α |aij | ≥ 2 |aii | , i ∈ α, j=1 P (A/A (α)) = A (β) + A (β, α) [ρ (A) I − A (α)]−1 A (α, β) is a nonnegative irreducible H-matrix. PROOF. Let α = {i1 , i2 , ..., ik} and β = {j1 , j2 , ..., jl}, where k + l = n. Since A is an H-matrix, then there exists a positive diagonal matrix X = diag (x1 , x2 , ..., xn) > 0 such that X −1 AX is a strictly diagonally dominant matrix, i.e., xj |aii | > |aij | , i ∈ n . xi j=i Suppose that B = (bij ) = X −1 AX, we have ρ (B) = ρ (A) and B is a strictly diagonally dominant matrix. Since n ρ (A) ≥ maxi∈α |aij | ≥ 2 |aii | , i ∈ α j=1 and xj |aii | > |aij | , i ∈ n , xi j=i n we have ρ (B) = ρ (A) ≥ maxi∈α |bij |. Then,by Corollary 2.4, P (B/B (α)) is a j=1 strictly diagonally dominant matrix and x2 a11 x1 a12 · · · xn a1n x1 x1 a21 a22 · · · xn a2n B = x2 ··· x2 . ··· ··· ··· x1 x2 xn an1 xn an2 · · · ann Zeng et al. 295 Let D = diag (xj1 , xj2 , ..., xjl) > 0. Then, −1 P (B/B (α)) = B (β) + B (β, α) [ρ (B) I − B (α)] B (α, β) bj 1 j 1 · · · bj 1 j l bj 1 i 1 · · · bj 1 i k = ··· ··· ··· + ··· ··· ··· bj l j 1 · · · bj l j l bj l i 1 · · · bj l i k ρ (B) − bi1 i1 · · · −bi1 ik bi 1 j 1 · · · bi 1 j l × ··· ··· ··· ··· ··· ··· −bik i1 · · · ρ (B) − bik ik bi k j 1 · · · bi k j l xjl xi xi aj 1 j 1 · · · x j aj 1 j l xj1 aj1 i1 1 · · · xjk aj1 ik 1 1 = ··· ··· ··· + ··· ··· ··· xj1 xi1 xik xj aj l j 1 · · · aj l j l xjl aj l i 1 · · · x j aj l i k l xik xj l xj ρ (A) − ai1 i1 · · · − xi ai1 ik xi1 ai1 j1 1 · · · x i l ai 1 j l 1 1 × ··· ··· ··· ··· ··· ··· xi1 xj1 xjl − x i ai k i 1 · · · ρ (A) − aik ik xik ai k j 1 · · · x i ai k j l k k aj 1 j 1 · · · aj 1 j l 1 1 = diag , ..., ··· · · · · · · diag(xj1 , ..., xjl) xj1 xjl aj l j 1 · · · aj l j l a · · · aj 1 i k 1 1 j1 i1 +diag , ..., ··· · · · · · · diag(xi1 , ..., xik ) xj1 xjl aj l i k · · · aj l i k ρ (A) − ai1 i1 · · · −ai1 ik 1 1 ×diag , ..., ··· ··· ··· diag(xi1 , ..., xik ) xi1 xik −aik i1 · · · ρ (A) − aik ik a · · · ai 1 j l 1 1 i1 j1 ×diag , ..., ··· · · · · · · diag(xj1 , ..., xjl) xi1 xik ai k j 1 · · · ai k j l = D−1 A (β) D + D−1 A (β, α) [ρ (A) I − A (α)]−1 A (α, β) D = D−1 P (A/A (α)) D. Note that the matrix P (B/B (α)) = D−1 P (A/A (α)) D is strictly diagonally dominant, then P (A/A (α)) is an H-matrix. By Lemma 2.1, we have the matrix P (A/A (α)) is nonnegative irreducible. This completes the proof. 296 Perron Complements of Matrices 4 Example Let 3 1 1 0 1 3 0 1 A= 2 . 1 4 1 1 2 1 4 Obviously, A is a diagonally dominant and nonnegative irreducible H-matrix. And, n ρ (A) = 6.3028 ≥ maxi∈α |aij | ≥ 2 |aii | , i ∈ α, α = {1} or {1, 2}. j=1 Then, 3.3028 0.3028 1 P (A/A (α)) = 1.6055 4.6055 1 , 2.3028 1.3028 4 where α = {1}, is a diagonally dominant H-matrix. And, 4.7675 1.5351 P (A/A (α)) = , 1.5351 4.7675 where α = {1, 2}, is a diagonally dominant H-matrix. References [1] D. Carlson and T. Markham, Schur complements of diagonally dominant matrices, Czech. Math. J., 29(104)(1979), 246–251. [2] C. D. Meyer, Uncoupling the Perron eigenvector problem, Linear Algebra Appl., 114/115(1989), 69–94. [3] C. D. Meyer, Stochastic complementation uncoupling Markov chains and the theory of nearly reducible systems, SIAM Rev., 31(1989), 240–272. [4] M. Neumann, Inverses of Perron complements of inverse M-matrices, Linear Algebra Appl., 313(2000), 163–171.

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