
<p>Filomat 31:16 (2017), 5151–5158 https://doi.org/10.2298/FIL1716151X </p><p>Published by Faculty of Sciences and Mathematics, University of Nisˇ, Serbia </p><p>Available at: <a href="/goto?url=http://www.pmf.ni.ac.rs/filomat" target="_blank">http://www.pmf.ni.ac.rs/filomat </a></p><p><strong>The Drazin Inverse of the Sum of Two Matrices and its Applications </strong></p><p><strong>Lingling Xia</strong><sup style="top: -0.3013em;"><strong>a</strong></sup><strong>, Bin Deng</strong><sup style="top: -0.3013em;"><strong>a </strong></sup></p><p><sup style="top: -0.2411em;">a</sup>School of Mathematics, Hefei University of Technology, Hefei, 230009,China </p><p><strong>Abstract. </strong>In this paper, we give the results for the Drazin inverse of P + Q, then derive a representation </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>AC<br>BD</p><p></p><ul style="display: flex;"><li style="flex:1">for the Drazin inverse of a block matrix M = </li><li style="flex:1">under some conditions. Moreover, some alternative </li></ul><p>representations for the Drazin inverse of M<sup style="top: -0.2713em;">D </sup>where the generalized Schur complement S = D − CA<sup style="top: -0.2713em;">D</sup>B is nonsingular. Finally, the numerical example is given to illustrate our results. </p><p><strong>1. Introduction and preliminaries </strong></p><p>Let C<sup style="top: -0.3013em;">n×n </sup>denote the set of n × n complex matrix. By R(A), N(A) and rank(A) we denote the range, the null space and the rank of matrix A. The Drazin inverse of A is the unique matrix A<sup style="top: -0.3013em;">D </sup>satisfying </p><p></p><ul style="display: flex;"><li style="flex:1">A<sup style="top: -0.3013em;">D</sup>AA<sup style="top: -0.3013em;">D </sup>= A<sup style="top: -0.3013em;">D</sup>, </li><li style="flex:1">AA<sup style="top: -0.3013em;">D </sup>= A<sup style="top: -0.3013em;">D</sup>A, </li></ul><p></p><p>A<sup style="top: -0.3013em;">k+1</sup>A<sup style="top: -0.3013em;">D </sup>= A<sup style="top: -0.3013em;">k</sup>. </p><p>(1) where k = ind(A) is the index of A, the smallest nonnegative integer k which satisfies rank(A<sup style="top: -0.3013em;">k+1</sup>) = rank(A<sup style="top: -0.3013em;">k</sup>). If ind(A) = 0, then we call A<sup style="top: -0.3013em;">D </sup>is the group inverse of A and denote it by A<sup style="top: -0.3013em;">]</sup>. If ind(A) = 0, then A<sup style="top: -0.3013em;">D </sup>= A<sup style="top: -0.3013em;">−1</sup>. In addition, we denote A<sup style="top: -0.3013em;">π </sup>= I − AA<sup style="top: -0.3013em;">D</sup>, and define A<sup style="top: -0.3013em;">0 </sup>= I, where I is the identity matrix with proper sizes[1]. <br>For A ∈ C<sup style="top: -0.3014em;">n×n</sup>, k is the index of A, there exists unique matrices C and N, such that A = C + N, CN = NC = 0, <br>N is the nilpotent of index k, and ind(C) = 0 or 1.We shall always use C, N in this context and will refer to A = C + N as the core-nilpotent decomposition of A, Note that A<sup style="top: -0.3014em;">D </sup>= C<sup style="top: -0.3014em;">D</sup>. <br>The Drazin inverse of a square matrix is widely applied in many fields, such as singular differential or difference equations, Markov chains, iterative method, cryptography and numerical analysis,which can be found in[2, 3]. The Drazin inverse in perturbation bounds for the relative eigenvalue problem has an important application value [4]. Accordingly, the Drazin inverse of 2 × 2 block matrix and its applications can be found in [3]. <br>Suppose P, Q ∈ C<sup style="top: -0.3013em;">n×n </sup>such that PQ = QP = 0, then (P + Q)<sup style="top: -0.3013em;">D</sup>=P<sup style="top: -0.3013em;">D</sup>+Q<sup style="top: -0.3013em;">D</sup>. This result was firstly proved by <br>Drazin [5] in 1958. In 2001, Hartwig et al. [6] gave a formula for (P+Q)<sup style="top: -0.3013em;">D </sup>under the one side condition PQ = 0. In 2005, Castro-Gonza´lez [7] derived a result under the conditions P<sup style="top: -0.3013em;">D</sup>Q = 0, PQ<sup style="top: -0.3013em;">D </sup>= 0 and Q<sup style="top: -0.3013em;">π</sup>PQP<sup style="top: -0.3013em;">π </sup>= 0. In 2009, Mart´ınez-Serrano and Castro-Gonza´lez [8] extended these results to the case P<sup style="top: -0.3014em;">2</sup>Q = 0, Q<sup style="top: -0.3014em;">2 </sup>= 0 and gave the formula for (P + Q)<sup style="top: -0.3014em;">D</sup>. Hartwig and Patricio [9] under the condition P<sup style="top: -0.3014em;">2</sup>Q + PQ<sup style="top: -0.3014em;">2 </sup>= 0. In 2010, Wei and Deng [10] studied the additive result for generalized Drazin inverse under the commutative condition of PQ = QP on a Banach space. Liu et al. [11] gave the representations of the Drazin inverse of (P ± Q)<sup style="top: -0.3013em;">D </sup>with P<sup style="top: -0.3013em;">3</sup>Q = QP and Q<sup style="top: -0.3013em;">3</sup>P = PQ satisfied. In 2011, Liu et al. [12] extended the results to the case P<sup style="top: -0.3013em;">2</sup>Q = 0, QPQ = 0. In 2012, Bu et al. [13] gave the representations of the Drazin inverse of (P + Q)<sup style="top: -0.3013em;">D </sup>under the following conditions: </p><p>(i)P<sup style="top: -0.3428em;">2</sup>Q = 0, Q<sup style="top: -0.3428em;">2</sup>P = 0; (ii)QPQ = 0, QP<sup style="top: -0.3428em;">2</sup>Q = 0, P<sup style="top: -0.3428em;">3</sup>Q = 0. </p><p>2010 Mathematics Subject Classification. 15A09 </p><p>Keywords. Drazin inverse,Block matrix,Generalized Schur complement,Index Received: 25 September 2014; Revised: 07 January 2015; Accepted: 21 January 2015 Communicated by Dragana Cvetkovic´ - Ilic´ This work was supported by the Fundamental Research Funds for the Central Universities(J2014HGXJ0068) </p><p>Email address: <a href="mailto:[email protected]" target="_blank">[email protected] </a>(Bin Deng) </p><p>L. Xia, B. Deng / Filomat 31:16 (2017), 5151–5158 </p><p>5152 <br>The results about the representation of (P + Q)<sup style="top: -0.3013em;">D </sup>are useful in computing the representations of the Drazin inverse for block matrices, analyzing a class of perturbation and iteration theory. The general questions of how </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>D</p><p>AC<br>BD</p><p></p><ul style="display: flex;"><li style="flex:1">to express (P + Q)<sup style="top: -0.3014em;">D </sup>by P, Q, P<sup style="top: -0.3014em;">D</sup>, Q<sup style="top: -0.3014em;">D </sup>and </li><li style="flex:1">by A, B, C, D without side condition are very difficult and </li></ul><p>have not been solved. <br>In this paper, we first give the formulas of (P + Q)<sup style="top: -0.3014em;">D </sup>under the conditions P<sup style="top: -0.3014em;">2</sup>Q = 0, PQ + QP = 0 and </p><p>T</p><p>P<sup style="top: -0.3014em;">D</sup>Q = 0, PQ − QP = 0, N(P) N(Q) = 0. And similar reasoning is presented. In the second, we use the </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>AC<br>BD</p><p></p><ul style="display: flex;"><li style="flex:1">formulas of (P + Q)<sup style="top: -0.3013em;">D </sup>to give some representations for the Drazin inverse of block matrix M = </li><li style="flex:1">(A and </li></ul><p>D are square) under some conditions. Then we give the representation of M<sup style="top: -0.3013em;">D </sup>in which the generalized Schur complement S = D − CA<sup style="top: -0.3013em;">D</sup>B is nonsingular under new conditions. Finally, we take some numerical examples to illustrate our results. <br>Before giving the main results, we first introduce several lemmas as follows. </p><p><strong>Lemma 1.1. </strong>([14]) Let </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>AC</p><p>0</p><p>B<br>B</p><p>0</p><p>CA</p><p>M<sub style="top: 0.1246em;">1 </sub>= </p><p>,</p><p>M<sub style="top: 0.1246em;">2 </sub>= </p><p>,</p><p>where A and B are square matrices with ind(A)=r and ind(B)=s. Then </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>A<sup style="top: -0.3013em;">D </sup></p><p>X</p><p>0</p><p>B<sup style="top: -0.3013em;">D </sup></p><p>0</p><p>X</p><p>M<sup style="top: -0.3014em;">D</sup><sub style="top: 0.3069em;">1 </sub>= </p><p>,</p><p>M<sup style="top: -0.3014em;">D</sup><sub style="top: 0.2972em;">2 </sub>= </p><p>,</p><p></p><ul style="display: flex;"><li style="flex:1">B<sup style="top: -0.3013em;">D </sup></li><li style="flex:1">A<sup style="top: -0.3013em;">D </sup></li></ul><p></p><p></p><ul style="display: flex;"><li style="flex:1">P</li><li style="flex:1">P</li></ul><p></p><p></p><ul style="display: flex;"><li style="flex:1">r−1 </li><li style="flex:1">s−1 </li></ul><p></p><p>where X = <sub style="top: 0.2178em;">i=0 </sub>(B<sup style="top: -0.3013em;">D</sup>)<sup style="top: -0.3013em;">i+2</sup>CA<sup style="top: -0.3013em;">i</sup>A<sup style="top: -0.3013em;">π </sup>+ B<sup style="top: -0.3013em;">π </sup><sub style="top: 0.2178em;">i=0 </sub>B<sup style="top: -0.3013em;">i</sup>C(A<sup style="top: -0.3013em;">D</sup>)<sup style="top: -0.3013em;">i+2 </sup>− B<sup style="top: -0.3013em;">D</sup>CA<sup style="top: -0.3013em;">D</sup>. <strong>Lemma 1.2. </strong>([6]) Let P, Q ∈ C<sup style="top: -0.3013em;">n×n </sup>be such that ind(P)=r, ind(Q)=s and PQ = 0. Then </p><p></p><ul style="display: flex;"><li style="flex:1">s−1 </li><li style="flex:1">r−1 </li></ul><p></p><p></p><ul style="display: flex;"><li style="flex:1">X</li><li style="flex:1">X</li></ul><p></p><p>(P + Q)<sup style="top: -0.3013em;">D </sup>= Q<sup style="top: -0.3013em;">π </sup>Q<sup style="top: -0.3429em;">i</sup>(P<sup style="top: -0.3429em;">D</sup>)<sup style="top: -0.3429em;">i+1 </sup></p><p>+</p><p>(Q<sup style="top: -0.3429em;">D</sup>)<sup style="top: -0.3429em;">i+1</sup>P<sup style="top: -0.3429em;">i</sup>P<sup style="top: -0.3429em;">π</sup>. </p><p></p><ul style="display: flex;"><li style="flex:1">i=0 </li><li style="flex:1">i=0 </li></ul><p></p><p><strong>Lemma 1.3. </strong>([8]) Let A, B ∈ C<sup style="top: -0.3014em;">n×n </sup></p><p>,<br>(i) If R is nonsingular and B = RAR<sup style="top: -0.3013em;">−1</sup>, then B<sup style="top: -0.3013em;">D </sup>= RA<sup style="top: -0.3013em;">D</sup>R<sup style="top: -0.3013em;">−1</sup>. </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>A<sub style="top: 0.1246em;">1 </sub></p><p>0<br>0</p><p>A<sub style="top: 0.1246em;">2 </sub></p><p></p><ul style="display: flex;"><li style="flex:1">(ii) If ind(A)=k ≥ 0, then exists a nonsingular matrix R such that A = R </li><li style="flex:1">R<sup style="top: -0.3014em;">−1</sup>, where A<sub style="top: 0.1245em;">1 </sub>∈ C<sup style="top: -0.3014em;">r×r </sup>is </li></ul><p>nonsingular and A<sub style="top: 0.1246em;">2 </sub>∈ C<sup style="top: -0.3013em;">(n−r)×(n−r) </sup>is k-nilpotent. Relative to the above form, A<sup style="top: -0.3013em;">D </sup>and A<sup style="top: -0.3013em;">π </sup>= I − AA<sup style="top: -0.3013em;">D</sup>, are given by </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>A<sup style="top: -0.3014em;">−</sup><sub style="top: 0.3058em;">1 </sub><sup style="top: -0.3014em;">1 </sup></p><p>0<br>00<br>00<br>0</p><p>I<br>A<sup style="top: -0.3014em;">D </sup>= R </p><p>R<sup style="top: -0.3014em;">−1</sup>, </p><p>A<sup style="top: -0.3014em;">π </sup>= R </p><p>R<sup style="top: -0.3014em;">−1</sup>. </p><p><strong>Lemma 1.4. </strong>([5]) Let P, Q ∈ C<sup style="top: -0.3013em;">n×n </sup>be such that PQ = QP = 0, then (P + Q)<sup style="top: -0.3013em;">D </sup>= P<sup style="top: -0.3013em;">D </sup>+ Q<sup style="top: -0.3013em;">D</sup>. </p><p></p><ul style="display: flex;"><li style="flex:1">ꢀ</li><li style="flex:1">ꢁ</li></ul><p></p><p><strong>Lemma 1.5. </strong>([1]) Let A ∈ C<sup style="top: -0.3014em;">m×n</sup>, B ∈ C<sup style="top: -0.3014em;">n×m</sup>, then (AB)<sup style="top: -0.3014em;">D </sup>= A (BA)<sup style="top: -0.3014em;">2 </sup><sup style="top: -0.6362em;">D</sup>B. <strong>Lemma 1.6. </strong>([1]) Let A, B ∈ C<sup style="top: -0.3013em;">n×n</sup>, if AB = BA, then </p><p>(i) (AB)<sup style="top: -0.3013em;">D </sup>= B<sup style="top: -0.3013em;">D</sup>A<sup style="top: -0.3013em;">D </sup>= A<sup style="top: -0.3013em;">D</sup>B<sup style="top: -0.3013em;">D</sup>. (ii) A<sup style="top: -0.3014em;">D</sup>B = BA<sup style="top: -0.3014em;">D </sup>and AB<sup style="top: -0.3014em;">D </sup>= B<sup style="top: -0.3014em;">D</sup>A. </p><p><strong>Lemma 1.7. </strong>([20]) Let A, B ∈ C<sup style="top: -0.3014em;">n×n</sup>, suppose that c is such that (cA + B) is invertible, then <br>(i) (cA + B)<sup style="top: -0.3014em;">−1</sup>A and (cA + B)<sup style="top: -0.3014em;">−1</sup>B commute; </p><p></p><ul style="display: flex;"><li style="flex:1">ꢀ</li><li style="flex:1">ꢁ</li><li style="flex:1">ꢀ</li><li style="flex:1">ꢁ</li></ul><p></p><p>(ii) N(A) ∩ N(B) = {0} and N(A) = N (cA + B)<sup style="top: -0.3013em;">−1</sup>A , N(B) = N (cA + B)<sup style="top: -0.3013em;">−1</sup>B . </p><p>L. Xia, B. Deng / Filomat 31:16 (2017), 5151–5158 </p><p>5153 </p><p><strong>2. Additive Results </strong></p><p>In [10], Wei and Deng studied the additive result for generalized Drazin inverse under the commutative condition of PQ = QP on a Banach space. In this section, we will give the Drazin inverse of P + Q under the conditions that P<sup style="top: -0.3013em;">2</sup>Q = 0, PQ + QP = 0 and P<sup style="top: -0.3013em;">D</sup>Q = 0, PQ − QP = 0, N(P) ∩ N(Q) = {0}, which will be the main tool in our following development. </p><p><strong>Theorem 2.1. </strong>Let P, Q ∈ C<sup style="top: -0.3013em;">n×n </sup>be such that P<sup style="top: -0.3013em;">2</sup>Q = 0, PQ + QP = 0, then </p><p>(P + Q)<sup style="top: -0.3013em;">D </sup>= P<sup style="top: -0.3013em;">D </sup>+ (P + Q)(Q<sup style="top: -0.3013em;">D</sup>)<sup style="top: -0.3013em;">2</sup>. </p><p>(2) <br>Proo f. From the conditions of theorem , we can know P<sup style="top: -0.3013em;">2</sup>Q = −PQP = QP<sup style="top: -0.3013em;">2 </sup>= 0. </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>P<sub style="top: 0.1245em;">1 </sub></p><p>0</p><p>P<sub style="top: 0.1246em;">2 </sub><br>Q<sub style="top: 0.1245em;">1 </sub>Q<sub style="top: 0.1245em;">12 </sub>Q<sub style="top: 0.1246em;">21 </sub>Q<sub style="top: 0.1246em;">2 </sub></p><p></p><ul style="display: flex;"><li style="flex:1">Let P = R </li><li style="flex:1">R<sup style="top: -0.3013em;">−1</sup>, where P<sub style="top: 0.1246em;">1 </sub>is nonsingular and P<sub style="top: 0.1246em;">2 </sub>is nilpotent. Write Q = R </li><li style="flex:1">R<sup style="top: -0.3013em;">−1</sup>. Form </li></ul><p>0</p><p>P<sup style="top: -0.3013em;">2</sup>Q = 0 it follows </p><p>Q<sub style="top: 0.1246em;">1 </sub>= 0, Q<sub style="top: 0.1246em;">12 </sub>= 0, P<sup style="top: -0.3013em;">2</sup><sub style="top: 0.2962em;">2</sub>Q<sub style="top: 0.1246em;">21 </sub>= 0, P<sub style="top: 0.2962em;">2</sub><sup style="top: -0.3013em;">2</sup>Q<sub style="top: 0.1246em;">2 </sub>= 0 </p><p>Form QP<sup style="top: -0.3014em;">2 </sup>= 0 it follows <br>(3) </p><p>Q<sub style="top: 0.1245em;">21 </sub>= 0, Q<sub style="top: 0.1245em;">2</sub>P<sup style="top: -0.3014em;">2</sup><sub style="top: 0.2962em;">2 </sub>= 0. </p><p>(4) (5) <br>Form PQ + QP = 0 it follows P<sub style="top: 0.1246em;">2</sub>Q<sub style="top: 0.1246em;">2 </sub>+ Q<sub style="top: 0.1246em;">2</sub>P<sub style="top: 0.1246em;">2 </sub>= 0. Now, using Lemma 1.1 we obtain </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p></p><ul style="display: flex;"><li style="flex:1">D</li><li style="flex:1">D</li></ul><p></p><p>P<sub style="top: 0.1246em;">1 </sub></p><p>0<br>0</p><p>P<sup style="top: -0.3013em;">−</sup><sub style="top: 0.3058em;">1 </sub><sup style="top: -0.3013em;">1 </sup></p><p>0<br>0</p><p></p><ul style="display: flex;"><li style="flex:1">(P + Q)<sup style="top: -0.3014em;">D </sup>= R </li><li style="flex:1">R<sup style="top: -0.3014em;">−1 </sup>= R </li></ul><p></p><p>R<sup style="top: -0.3014em;">−1</sup>. </p><p></p><ul style="display: flex;"><li style="flex:1">(P<sub style="top: 0.1245em;">2 </sub>+ Q<sub style="top: 0.1245em;">2</sub>)<sup style="top: -0.3013em;">D </sup></li><li style="flex:1">P<sub style="top: 0.1246em;">2 </sub>+ Q<sub style="top: 0.1246em;">2 </sub></li></ul><p></p><p>Now, we need compute (P<sub style="top: 0.1245em;">2 </sub>+ Q<sub style="top: 0.1245em;">2</sub>)<sup style="top: -0.3014em;">D</sup>. </p><p>(P<sub style="top: 0.1245em;">2 </sub>+ Q<sub style="top: 0.1245em;">2</sub>)<sup style="top: -0.3014em;">2 </sup>= P<sub style="top: 0.2961em;">2</sub><sup style="top: -0.3014em;">2 </sup>+ Q<sub style="top: 0.2961em;">2</sub><sup style="top: -0.3014em;">2 </sup>+ P<sub style="top: 0.1245em;">2</sub>Q<sub style="top: 0.1245em;">2 </sub>+ Q<sub style="top: 0.1245em;">2</sub>P<sub style="top: 0.1245em;">2 </sub>= P<sup style="top: -0.3014em;">2</sup><sub style="top: 0.2961em;">2 </sub>+ Q<sup style="top: -0.3014em;">2</sup><sub style="top: 0.2961em;">2</sub>, (P<sup style="top: -0.3014em;">2</sup><sub style="top: 0.2961em;">2</sub>)<sup style="top: -0.3014em;">D </sup>= (P<sub style="top: 0.2972em;">2</sub><sup style="top: -0.3014em;">D</sup>)<sup style="top: -0.3014em;">2 </sup>= 0. </p><p>Applying (3) and (4) and Lemma 1.4, we get </p><p></p><ul style="display: flex;"><li style="flex:1">ꢀ</li><li style="flex:1">ꢁ</li></ul><p></p><p>D</p><p>(P<sub style="top: 0.1246em;">2 </sub>+ Q<sub style="top: 0.1246em;">2</sub>)<sup style="top: -0.3013em;">2 </sup>= (P<sup style="top: -0.3013em;">2</sup><sub style="top: 0.2962em;">2 </sub>+ Q<sup style="top: -0.3013em;">2</sup><sub style="top: 0.2962em;">2</sub>)<sup style="top: -0.3013em;">D </sup>= (Q<sup style="top: -0.3013em;">2</sup><sub style="top: 0.2962em;">2</sub>)<sup style="top: -0.3013em;">D</sup>. </p><p>Further, </p><p></p><ul style="display: flex;"><li style="flex:1">ꢀ</li><li style="flex:1">ꢁ</li></ul><p></p><p>D</p><p>(P<sub style="top: 0.1245em;">2 </sub>+ Q<sub style="top: 0.1245em;">2</sub>)<sup style="top: -0.3013em;">D </sup>= (P<sub style="top: 0.1245em;">2 </sub>+ Q<sub style="top: 0.1245em;">2</sub>) (P<sub style="top: 0.1245em;">2 </sub>+ Q<sub style="top: 0.1245em;">2</sub>)<sup style="top: -0.3013em;">2 </sup>= (P<sub style="top: 0.1245em;">2 </sub>+ Q<sub style="top: 0.1245em;">2</sub>)(Q<sup style="top: -0.3013em;">2</sup><sub style="top: 0.2962em;">2</sub>)<sup style="top: -0.3013em;">D</sup>. </p><p>(6) <br>By substituting (6) in (5), we get </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>P<sub style="top: 0.1246em;">1 </sub></p><p>0<br>00<br>00<br>0</p><p></p><ul style="display: flex;"><li style="flex:1">(P + Q)<sup style="top: -0.3014em;">D </sup>= R </li><li style="flex:1">R<sup style="top: -0.3013em;">−1 </sup>+ R </li><li style="flex:1">R<sup style="top: -0.3013em;">−1 </sup>= P<sup style="top: -0.3013em;">D </sup>+ (P + Q)(Q<sup style="top: -0.3013em;">D</sup>)<sup style="top: -0.3013em;">2</sup>. </li></ul><p></p><p>(P<sub style="top: 0.1245em;">2 </sub>+ Q<sub style="top: 0.1245em;">2</sub>)<sup style="top: -0.3014em;">D </sup></p><p>Using the similar method as in the proof of Theorem 2.1, We get the following two results. </p><p><strong>Theorem 2.2. </strong>Let P, Q ∈ C<sup style="top: -0.3014em;">n×n </sup>be such that P<sup style="top: -0.3014em;">2</sup>Q = 0, QP<sup style="top: -0.3014em;">π </sup>= 0, then </p><p>(P + Q)<sup style="top: -0.3013em;">D </sup>= P<sup style="top: -0.3013em;">D </sup>+ Q(P<sup style="top: -0.3013em;">D</sup>)<sup style="top: -0.3013em;">2 </sup>+ PQ(P<sup style="top: -0.3013em;">D</sup>)<sup style="top: -0.3013em;">3</sup>. </p><p>(7) (8) </p><p><strong>Theorem 2.3. </strong>Let P, Q ∈ C<sup style="top: -0.3013em;">n×n </sup>be such that P<sup style="top: -0.3013em;">D</sup>Q = 0, QP<sup style="top: -0.3013em;">π </sup>= 0, s = indP, then </p><p>s−1 </p><p>X</p><p>(P + Q)<sup style="top: -0.3014em;">D </sup>= P<sup style="top: -0.3014em;">D </sup>+ </p><p>P<sup style="top: -0.3429em;">i</sup>Q(P<sup style="top: -0.3429em;">D</sup>)<sup style="top: -0.3429em;">i+2 </sup></p><p>.</p><p>i=0 </p><p>L. Xia, B. Deng / Filomat 31:16 (2017), 5151–5158 </p><p>5154 <br>(9) </p><p><strong>Theorem 2.4. </strong>Let P, Q ∈ C<sup style="top: -0.3013em;">n×n </sup>be such that PQ = QP, P<sup style="top: -0.3013em;">D</sup>Q = 0, N(P) ∩ N(Q) = {0}, then </p><p>k−1 </p><p>X</p><p>(P + Q)<sup style="top: -0.3014em;">D </sup>= P<sup style="top: -0.3014em;">D </sup>+ </p><p>(Q<sup style="top: -0.3429em;">D</sup>)<sup style="top: -0.3429em;">n+1</sup>(−P)<sup style="top: -0.3429em;">n</sup>. </p><p>n=0 </p><p>where k = ind(P). </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>P<sub style="top: 0.1246em;">1 </sub></p><p>0<br>0</p><p>P<sub style="top: 0.1245em;">2 </sub></p><p>Proo f. Let P = R </p><p>R<sup style="top: -0.3014em;">−1</sup>, where P<sub style="top: 0.1245em;">1 </sub>and R is nonsingular, and P<sub style="top: 0.1245em;">2 </sub>is nilpotent, then exists a positive real </p><p></p><ul style="display: flex;"><li style="flex:1"> </li><li style="flex:1">!</li></ul><p></p><p>Q<sub style="top: 0.1245em;">1 </sub>Q<sub style="top: 0.1245em;">12 </sub>Q<sub style="top: 0.1245em;">21 </sub>Q<sub style="top: 0.1245em;">2 </sub></p><p>number k, satisfy P<sup style="top: -0.3013em;">k</sup><sub style="top: 0.2972em;">2 </sub>= 0, and we can easy know k = ind(P). Write Q = R follows Q<sub style="top: 0.1245em;">1 </sub>= 0, Q<sub style="top: 0.1245em;">2 </sub>= 0. <br>R<sup style="top: -0.3013em;">−1</sup>. From P<sup style="top: -0.3013em;">D</sup>Q = 0 it <br>Now, from PQ = QP it follows P<sub style="top: 0.1245em;">2</sub>Q<sub style="top: 0.1245em;">3 </sub>= Q<sub style="top: 0.1245em;">3</sub>P<sub style="top: 0.1245em;">1</sub>, P<sub style="top: 0.1245em;">2</sub>Q<sub style="top: 0.1245em;">4 </sub>= Q<sub style="top: 0.1245em;">4</sub>P<sub style="top: 0.1245em;">2</sub>. Then P<sup style="top: -0.3013em;">k</sup><sub style="top: 0.2972em;">2</sub>Q<sub style="top: 0.1246em;">3 </sub>= Q<sub style="top: 0.1246em;">3</sub>P<sub style="top: 0.3069em;">1</sub><sup style="top: -0.3013em;">k </sup>= 0. Thus Q<sub style="top: 0.1246em;">3 </sub>= 0 since P<sub style="top: 0.3069em;">1</sub><sup style="top: -0.3013em;">k </sup>is invertible. Next we will show that Q<sub style="top: 0.1246em;">4 </sub>is invertible. If P<sub style="top: 0.1245em;">2 </sub>= 0, the assumption N(P) ∩ N(Q) = {0} implies N(Q<sub style="top: 0.1245em;">4</sub>) = {0}, and we are done. If P<sub style="top: 0.1245em;">2 </sub>, 0, suppose there exists a v , 0 such that v ∈ N(Q<sub style="top: 0.1246em;">4</sub>). Then </p>
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