Filomat 31:16 (2017), 5151–5158 Published by Faculty of Sciences and Mathematics, https://doi.org/10.2298/FIL1716151X University of Nis,ˇ Serbia Available at: http://www.pmf.ni.ac.rs/filomat The Drazin Inverse of the Sum of Two Matrices and its Applications Lingling Xiaa, Bin Denga aSchool of Mathematics, Hefei University of Technology, Hefei, 230009,China Abstract. In this paper, we give the results for the Drazin inverse of P + Q, then derive a representation ! AB for the Drazin inverse of a block matrix M = under some conditions. Moreover, some alternative CD representations for the Drazin inverse of MD where the generalized Schur complement S = D CADB is − nonsingular. Finally, the numerical example is given to illustrate our results. 1. Introduction and preliminaries n n Let C × denote the set of n n complex matrix. By (A), (A) and rank(A) we denote the range, the null space and the rank of matrix ×A. The Drazin inverse of AR is theN unique matrix AD satisfying ADAAD = AD; AAD = ADA; Ak+1AD = Ak: (1) where k = ind(A) is the index of A, the smallest nonnegative integer k which satisfies rank(Ak+1) = rank(Ak). If D ] D 1 ind(A) = 0, then we call A is the group inverse of A and denote it by A . If ind(A) = 0, then A = A− . In addition, we denote Aπ = I AAD, and define A0 = I, where I is the identity matrix with proper sizes[1]. n n − For A C × , k is the index of A, there exists unique matrices C and N, such that A = C + N, CN = NC = 0, N is the nilpotent2 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 AD = CD. 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]. × n n D D D Suppose P; Q C × such that PQ = QP = 0, then (P + Q) =P +Q . This result was firstly proved by Drazin [5] in 1958.2 In 2001, Hartwig et al. [6] gave a formula for (P+Q)D under the one side condition PQ = 0. In 2005, Castro-Gonzalez´ [7] derived a result under the conditions PDQ = 0, PQD = 0 and QπPQPπ = 0. In 2009, Mart´ınez-Serrano and Castro-Gonzalez´ [8] extended these results to the case P2Q = 0; Q2 = 0 and gave the formula for (P + Q)D. Hartwig and Patricio [9] under the condition P2Q + PQ2 = 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)D with P3Q = QP and Q3P = PQ satisfied. In 2011, Liu et al. [12] extended the results to the case P2Q = 0; QPQ± = 0. In 2012, Bu et al. [13] gave the representations of the Drazin inverse of (P + Q)D under the following conditions: (i)P2Q = 0; Q2P = 0; (ii)QPQ = 0; QP2Q = 0; P3Q = 0: 2010 Mathematics Subject Classification. 15A09 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) Email address: [email protected] (Bin Deng) L. Xia, B. Deng / Filomat 31:16 (2017), 5151–5158 5152 The results about the representation of (P + Q)D 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 !D AB to express (P + Q)D by P; Q; PD; QD and by A; B; C; D without side condition are very difficult and CD have not been solved. In this paper, we first give the formulas of (P + Q)D under the conditions P2Q = 0; PQ + QP = 0 and T PDQ = 0; PQ QP = 0; (P) (Q) = 0. And similar reasoning is presented. In the second, we use the − N N ! AB formulas of (P + Q)D to give some representations for the Drazin inverse of block matrix M = (A and CD D are square) under some conditions. Then we give the representation of MD in which the generalized Schur complement S = D CADB is nonsingular under new conditions. Finally, we take some numerical examples to illustrate our results.− Before giving the main results, we first introduce several lemmas as follows. Lemma 1.1. ([14]) Let ! ! A 0 BC M = ,M = ; 1 CB 2 0 A where A and B are square matrices with ind(A)=r and ind(B)=s. Then ! ! AD 0 BD X MD = ,MD = ; 1 XBD 2 0 AD Pr 1 D i+2 i π π Ps 1 i D i+2 D D where X = − (B ) CA A + B − B C(A ) B CA . i=0 i=0 − n n Lemma 1.2. ([6]) Let P; Q C × be such that ind(P)=r, ind(Q)=s and PQ = 0. Then 2 s 1 r 1 X− X− (P + Q)D = Qπ Qi(PD)i+1 + (QD)i+1PiPπ: i=0 i=0 n n Lemma 1.3. ([8]) Let A; B C × , 2 1 D D 1 (i) If R is nonsingular and B = RAR− , then B = RA R− . ! A1 0 1 r r (ii) If ind(A)=k 0, then exists a nonsingular matrix R such that A = R R− , where A1 C × is ≥ 0 A2 2 (n r) (n r) D π D nonsingular and A C − × − is k-nilpotent. Relative to the above form, A and A = I AA , are given by 2 2 − ! ! A 1 0 0 0 AD = R 1− R 1; Aπ = R R 1: 0 0 − 0 I − n n D D D Lemma 1.4. ([5]) Let P; Q C × be such that PQ = QP = 0, then (P + Q) = P + Q . 2 D m n n m D 2 Lemma 1.5. ([1]) Let A C × ; B C × , then (AB) = A (BA) B. 2 2 n n Lemma 1.6. ([1]) Let A; B C × , if AB = BA, then (i)(AB)D = BDAD = AD2BD. (ii) ADB = BAD and ABD = BDA. n n Lemma 1.7. ([20]) Let A; B C × , suppose that c is such that (cA + B) is invertible, then 1 2 1 (i)(cA + B)− A and (cA + B)− B commute; 1 1 (ii) (A) (B) = 0 and (A) = (cA + B)− A , (B) = (cA + B)− B . N \N f g N N N N L. Xia, B. Deng / Filomat 31:16 (2017), 5151–5158 5153 2. Additive Results 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 P2Q = 0; PQ + QP = 0 and PDQ = 0; PQ QP = 0; (P) (Q) = 0 , which will be the main tool in our following development. − N \N f g n n 2 Theorem 2.1. Let P; Q C × be such that P Q = 0; PQ + QP = 0, then 2 (P + Q)D = PD + (P + Q)(QD)2: (2) Proo f: From the conditions of theorem , we can know P2Q = PQP = QP2 = 0. − ! ! P1 0 1 Q1 Q12 1 Let P = R R− , where P1 is nonsingular and P2 is nilpotent. Write Q = R R− . Form 0 P2 Q21 Q2 P2Q = 0 it follows 2 2 Q1 = 0; Q12 = 0; P2Q21 = 0; P2Q2 = 0 (3) Form QP2 = 0 it follows 2 Q21 = 0; Q2P2 = 0: (4) Form PQ + QP = 0 it follows P2Q2 + Q2P2 = 0: Now, using Lemma 1.1 we obtain !D 1 !D D P1 0 1 P1− 0 1 (P + Q) = R R− = R D R− : (5) 0 P2 + Q2 0 (P2 + Q2) D Now, we need compute (P2 + Q2) : 2 2 2 2 2 2 D D 2 (P2 + Q2) = P2 + Q2 + P2Q2 + Q2P2 = P2 + Q2; (P2) = (P2 ) = 0: Applying (3) and (4) and Lemma 1:4, we get D 2 2 2 D 2 D (P2 + Q2) = (P2 + Q2) = (Q2) : Further, D D 2 2 D (P2 + Q2) = (P2 + Q2) (P2 + Q2) = (P2 + Q2)(Q2) : (6) By substituting (6) in (5), we get ! ! D P1 0 1 0 0 1 D D 2 (P + Q) = R R− + R D R− = P + (P + Q)(Q ) : 0 0 0 (P2 + Q2) Using the similar method as in the proof of Theorem 2.1, We get the following two results. n n 2 π Theorem 2.2. Let P; Q C × be such that P Q = 0; QP = 0, then 2 (P + Q)D = PD + Q(PD)2 + PQ(PD)3: (7) n n D π Theorem 2.3. Let P; Q C × be such that P Q = 0; QP = 0; s = indP, then 2 s 1 X− (P + Q)D = PD + PiQ(PD)i+2: (8) i=0 L. Xia, B. Deng / Filomat 31:16 (2017), 5151–5158 5154 n n D Theorem 2.4. Let P; Q C × be such that PQ = QP; P Q = 0; (P) (Q) = 0 , then 2 N \N f g k 1 X− (P + Q)D = PD + (QD)n+1( P)n: (9) − n=0 where k = ind(P).