Formulae for the generalized Drazin inverse of a block matrix in terms of Banachiewicz Schur forms

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1 Formulae for the generalized Drazin inverse of a block matrix in terms of Banachiewicz Schur forms Dijana Mosić and Dragan S Djordjević Abstract We introduce new expressions for the generalized Drazin inverse of a block matrix with the generalized Schur complement being generalized Drazin invertible in a Banach algebra under some conditions We generalized some recent results for Drazin inverse and group inverse of complex matrices Key words and phrases: generalized Drazin inverse, Schur complement, Banachiewicz Schur form, block matrix 2010 Mathematics subject classification: 46H05, 47A05, 15A09 1 Introduction The research on the representations of the Drazin inverse for block matrices is an important problem in the theory and applications of generalized inverses of matrices (see 2, 8, 11, 12, 15, 16) Generalized inverses of block matrices have important applications in automatics, probability, statistics, mathematical programming, numerical analysis, game theory, econometrics, control theory and so on 3, 4 In 1979, Campbell and Meyer proposed the problem of finding a formula for the Drazin inverse of a 2 2 matrix in terms of its various blocks, where the blocks on the diagonal are required to be square matrices 4 At the present time, there is not known a complete solution to this problem Let A be a complex unital Banach algebra with unit 1 For a A, let σ(a) be the spectrum of a We denote by A nil and A qnil the sets of all nilpotent and quasinilpotent elements (σ(a) = {0}) of A, respectively The author are supported by the Ministry of Education and Science, Republic of Serbia, grant no

2 Recall that an element b A is the generalized Drazin inverse (or Koliha Drazin inverse) of a A provided that bab = b, ab = ba, a a 2 b A qnil If the generalized Drazin inverse of a exists, it is unique and denoted by a d 13, and a is generalized Drazin invertible By A d we denote the set of all generalized Drazin invertible elements of A If a A d, then the spectral idempotent a of a corresponding to the set {0} is given by a = 1 aa d A particular case of the generalized Drazin inverse is the (ordinary) Drazin inverse for which a a 2 b A nil The group inverse is the Drazin inverse for which the condition a a 2 b A nil is replaced with a = aba By a # we denote the group inverse of a The set of all group invertible elements of A is denoted by A # We use the next lemma, recalling that part (i) is proved by Castro González and Koliha 5, and part (ii) for bounded linear operators is proved in 9 Lemma 11 Let b A d and a A qnil (i) 5, Corollary 34 If ab = 0, then a+b A d and (a+b) d = (b d ) n+1 a n (ii) 9, Theorem 22 If ba = 0, then a+b A d and (a+b) d = a n (b d ) n+1 If a A, and if p = p 2 A is an idempotent, then a has the following block matrix representation a11 a a = 12, a 21 a 22 where a 11 = pap, a 12 = pa(1 p), a 21 = (1 p)ap, a 22 = (1 p)a(1 p) Let a b x = A (1) c d relative to the idempotent p A, a (pap) d, and let the generalized Schur complement be denoted by s = d ca d b ((1 p)a(1 p)) d Notice that if pap is invertible in pap, then s becomes the (ordinary) Schur complement (20) The following useful lemma is proved in 14 for elements of a Banach algebra 2

3 Lemma 12 14, Lemma 21 Let x be defined as in (1) Then the following statements are equivalent: (i) x A d and x d = r, where a r = d + a d bs d ca d a d bs d s d ca d s d aa (ii) a b = bs, s c = ca bs and y = ca ss A qnil ; (2) The expression (2) is called the generalized Banachiewicz Schur form of x For more detailed see 1, 2, 6, 7, 10, 18 A general formula for block triangular matrices was given by Meyer and Rose 15, and Hartwig and Shoaf 12 Hence, a challenge in this field is to extend the result of Meyer and Rose to the case of arbitrary matrices Under different conditions, Castro-González and Martínez-Serrano 6 presented several explicit representations for the Drazin inverse of a block matrix with the group invertible generalized Schur complement Deng and Wei 8 studied conditions under which the Drazin inverse of block-operator matrix having generalized Schur complement Drazin invertible, can be expressed in terms of a matrix in the Banachiewicz-Schur form and its powers In this paper, we presente new explicit formulae for the generalized Drazin inverse of a block matrix x defined in (1) in the case that the generalized Schur complement is generalized Drazin invertible These expressions involve the generalized Banachiewicz-Schur form (2) Several special cases are analyzed: we notice that some of them extend results from 6, 17, 19 to more general settings 2 Generalized Drazin inverse Throughout this paper, we assume that a block matrix x is defined as in (1), where a (pap) d, and the generalized Schur complement satisfies s = d ca d b ((1 p)a(1 p)) d In the following theorem we provide the explicit expression for the generalized Drazin inverse of block matrix x in (1) in terms of a d, s d, and the generalized Banachiewicz Schur forms r defined as in (2) Theorem 21 Let x be defined as in (1), and let r be defined as in (2) If ca = 0 and abs = 0, (3) 3

4 then x A d and x d aa bs = r + n 0 a bss d 0 ss s c s ca d bss d Proof Assume that x = y + z, where aa bs y = aa bs ca ds = 0 ss r n+2 (4) a and z = 2 a d bss d caa d dss d Using equalities a a d = 0 and (3), we get zy = 0 Since aa (pap) qnil, ss ((1 p)a(1 p)) qnil and σ(y) σ pap (aa ) σ (1 p)a(1 p) (ss ), we conclude that y A qnil In order to show that z A d, let z = z 1 + z 2, where z 1 = a 2 a d ss d caa d aa d bss d ss d dss d and z 2 = 0 a bss d s caa d s ca d bss d Then z 1 z 2 = 0 and z2 2 = 0, ie z 2 A nil If we set A z1 a 2 a d, B z1 aa d bss d, C z1 ss d caa d and D z1 ss d dss d, we have A z1 (pap) #, A # z 1 = a d, A z 1 B z1 = a aa d bss d = 0, C z1 A z 1 = 0, S z1 = D z1 C z1 A # z 1 B z1 = s 2 s d ((1 p)a(1 p)) #, S z # 1 = s d, Sz 1 C z1 = s ss d caa d = 0, B z1 Sz 1 = 0 and Y z1 = 0 By Lemma 12, we deduce that z 1 A d and z1 d = r Applying Lemma 11(ii), we obtain that z A d and z d = r + z 2 r 2 Using Lemma 11(ii) again, x A d and x d = y n (z d ) n+1 Since z 1 z 2 = 0 and z d 1 = r, we have rz 2 = r 2 z 1 z 2 = 0 which gives x d = y n (1 + z 2 r)r n+1 From yr = 0 we obtain x d = (1 + z 2 r)r + y n z 2 r n+2 = r + y n z 2 r n+2 which yields (4) n=1 A geometrical reformulation of conditions ca = 0 and abs = 0 is as follows: (a d ) c and s (ab), where a = {x A : ax = 0} The following corollary is a straightforward application of Theorem 21 4

5 Corollary 21 Let x be defined as in (1), a (pap) # and let s = d ca # b ((1 p)a(1 p)) # If ca = 0 and abs = 0, then x A d and = bs # ca # a bs # bs s ca # r 1 p 1 + ca # 0 r 2 1; where a r 1 = # + a # bs # ca # a # bs # s # ca # s # If we assume that s b and a (sc), we have the following representation of generalized Drazin inverse x d Theorem 22 Let x be defined as in (1), and let r be defined as in (2) If bs = 0 and sca = 0, then x A d and x d aa n 0 = r + ca ss Proof Let x = y + z, where aa 0 y = ca ss 0 a b s caa d s ca d b a and z = 2 a d bss d caa d dss d r n+2 Then zy = 0 and we can finish the proof in the same way as in the proof of Theorem 21 By Theorem 22, we can verify the next expression for x d in the case when a and s are group invertible Corollary 22 Let x be defined as in (1) where a (pap) # and s = d ca # b ((1 p)a(1 p)) #, and let r 1 be defined as in Corollary 21 If bs = 0 and sca = 0, then x A d and = bs # ca # a bs # s ca # 1 p r ca b r 3 1 Using Corollary 21 and Corollary 22, we can verify the following result Corollary 23 Let x be defined as in (1), a (pap) # and s = d ca # b ((1 p)a(1 p)) #, and let r 1 be defined as in Corollary 21 If ca = 0 and bs = 0, then x A d and = bs # ca # a bs # s ca # r 1 p 1 5

6 Note that Corollary 23 recovers 6, Theorem 25 for the Drazin inverse of complex matrices If we assume that the generalized Schur complement s is invertible in Theorem 21 and Theorem 22, then we can prove the next result Corollary 24 Let x be defined as in (1), where a (pap) d and the generalized Schur complement satisfies s = d ca d b ((1 p)a(1 p)) 1 If ca = 0, then x A d and where x d = ( a n a b ) r2 n+1 r 2, a r 2 = d + a d bs 1 ca d a d bs 1 s 1 ca d s 1 Following the same strategy as in the proof of the preceding results, we derive a new representation for x d Theorem 23 Let x be defined as in (1), and let r be defined as in (2) If then x A d and x d = r + r n+2 a b = 0 and s ca = 0, Proof If we write x = y + z, where aa a y = b aa 0 s c s = d s c s s 0 bs ss d ca ss d ca d bs then we have that yz = 0 and y A qnil Suppose that z = z 1 + z 2, where a z 1 = 2 a d aa d bss d ss d caa d ss d dss d and z 2 = aa n 0 s c ss (5) a and z = 2 a d aa d b ss d c ss d d 0 aa d bs ss d ca ss d ca d bs Now we can check that z 2 z 1 = 0 and z2 2 = 0 Using Lemma 12 we get z 1 A d and z1 d = r From Lemma 11(i) it follows z Ad and z d = r + r 2 z 2 By Lemma 11(i), we conclude that x A d and x d = (z d ) n+1 y n = r + r n+2 z 2 y n implying (5) 6,

7 Observe that conditions a b = 0 and s ca = 0 are equivalent with the following geometrical conditions: ba aa and caa sa, where aa = {ax : x A} As a consequence of Theorem 23, we obtain the next result Corollary 25 Let x be defined as in (1), a (pap) #, s = d ca # b ((1 p)a(1 p)) #, and let r 1 be defined as in Corollary 21 If a b = 0 and s ca = 0, then x A d and = r # bs # ca 1 s # ca a # bs 1 p + r1 2 a # bs c 0 The following theorem gives a formula for the generalized Drazin inverse of x in (1) under assumptions ca sa and bsa aa Theorem 24 Let x be defined as in (1), and let r be defined as in (2) If then x A d and s c = 0 and a bs = 0, x d = r + r n+2 0 aa d bs ca ca d bs aa a b 0 ss n Proof Similarly as Theorem 23, we can prove this theorem using the representation x = y + z, where aa a y = b a 0 s and z = 2 a d aa d b s ss d c ss d d From Theorem 24, we can obtain the next corollary Corollary 26 Let x be defined as in (1) where a (pap) # and s = d ca # b ((1 p)a(1 p)) #, and let r 1 be defined as in Corollary 21 If s c = 0 and a bs = 0, then x A d and = r # bs # ca 1 s # ca a # bs 1 p + r1 3 0 ca b 7

8 Now, by Corollary 25 and Corollary 26, we prove the result which covers 6, Theorem 22 Corollary 27 Let x be defined as in (1), a (pap) #, s = d ca # b ((1 p)a(1 p)) #, and let r 1 be defined as in Corollary 21 If s c = 0 and a b = 0, then x A d and = r # bs # ca a # bs 1 s # ca 1 p Using Theorems 21 24, we can prove the next corollary which recovers 6, Corollary 23 and 17, Theorem 32 Corollary 28 Let x be defined as in (1) where a (pap) #, the generalized Schur complement satisfies s = d ca # b ((1 p)a(1 p)) #, and let r 1 be defined as in Corollary 21 If a b = 0 = bs and ca = 0 = s c, then x A d and x d = r 1 The next result is a consequence of Theorem 23 and Theorem 24 Corollary 29 Let x be defined as in (1), where a (pap) d and s = d ca d b ((1 p)a(1 p)) 1, and let r 2 be defined as in Corollary 24 If a b = 0, then x A d and x d = r 2 (1 + r2 n+1 ca a n 0 ) Next, by Corollary 24 and Corollary 29, we obtain an extension of the result for the Drazin inverse of a block matrix by Wei 19 to a block matrix of Banach algebra Corollary 210 Let x be defined as in (1), where a (pap) d and s = d ca d b ((1 p)a(1 p)) 1, and let r 2 be defined as in Corollary 24 If ca = 0 and a b = 0, then x A d and x d = r 2 Some representations for the generalized Drazin inverse of triangular matrices are presented in the following theorems a 0 Theorem 25 Let x =, where a (pap) c d d and s = d ((1 p)a(1 p)) d (i) If ca = 0, then x A d and x d = r 3 + s n s c 0 r n+2 3 ; 8

9 (i) If s c = 0, then x A d and x d = r 3 + r3 n+2 ca n a 0 ; where r 3 = a d 0 s d ca d s d Proof If we suppose that b = 0 in Theorem 21 and Theorem 24, we check these formulae If the hypothesis c = 0 is assumed in Theorem 22 and Theorem 23, we can verify the next result a b Theorem 26 Let x =, where a (pap) 0 d d and s = d ((1 p)a(1 p)) d (i) If bs = 0, then x A d and x d = r a n a b r n+2 4 ; (i) If a b = 0, then x A d and x d = r 4 + r4 n+2 0 bs s n ; where a d a r 4 = d bs d 0 s d Finally, we give an example to illustrate our results p 0 Example 21 Let A be a Banach algebra and let x = A c 1 p relative to the idempotent p A Hence, a d = a = p, a = 0, s = s d = 1 p, s = 0 and b = 0 Applying Theorem 21 or Theorem 25, we conclude that p 0 x A d and x d = c 1 p 9

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11 14 MZ Kolundžija, D Mosić, DS Djordjević, Further results on the generalized Drazin inverse of block matrices in Banach algebras, Bull Malays Math Sci Soc (to appear) 15 CD Meyer, NJ Rose, The index and the Drazin inverse of block triangular matrices, SIAM J Appl Math 33 (1977) D Mosić, DS Djordjević, Reprezentation for the generalized Drazin inverse of block matrices in Banach algebras, Appl Math Comput 218 (2012) X Sheng, G Chen, Some generalized inverses of partitioned matrix and quotient identity of generalized Schur complement, Appl Math Comput 196 (2008) Y Tian, Y Takane, More on generalized inverses of partitioned matrices with Banachiewicz-Schur forms, Linear Algebra Appl 430 (2009) Y Wei, Expressions for the Drazin inverse of 2 2 block matrix, Linear Multilinear Algebra 45 (1998) F Zhang (Ed), The Schur Complement and Its Applications, Springer, Berlin, 2005 Address: Faculty of Sciences and Mathematics, University of Niš, PO Box 224, Niš, Serbia D Mosić: dijana@pmfniacrs D S Djordjević: dragan@pmfniacrs 11