Hindawi Publishing Corporation Journal of Applied Mathematics Volume 2013, Article ID 271978, 5 pages http://dx.doi.org/10.1155/2013/271978 Research Article Completing a 2 × 2 Block Matrix of Real Quaternions with a Partial Specified Inverse Yong Lin1,2 and Qing-Wen Wang1 1 2 Department of Mathematics, Shanghai University, Shanghai 200444, China School of Mathematics and Statistics, Suzhou University, Suzhou 234000, China Correspondence should be addressed to Qing-Wen Wang; wqw858@yahoo.com.cn Received 4 December 2012; Revised 23 February 2013; Accepted 20 March 2013 Academic Editor: K. Sivakumar Copyright © 2013 Y. Lin and Q.-W. Wang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper considers a completion problem of a nonsingular 2 × 2 block matrix over the real quaternion algebra H: Let π1 , π2 , π1 , π2 be nonnegative integers, π1 + π2 = π1 + π2 = π > 0, and π΄ 12 ∈ Hπ1 ×π2 , π΄ 21 ∈ Hπ2 ×π1 , π΄ 22 ∈ Hπ2 ×π2 , π΅11 ∈ Hπ1 ×π1 be given. We determine necessary and sufficient conditions so that there exists a variant block entry matrix π΄ 11 ∈ Hπ1 ×π1 such that π΄ 12 π΄ = ( π΄π΄ 11 ) ∈ Hπ×π is nonsingular, and π΅11 is the upper left block of a partitioning of π΄−1 . The general expression for π΄ 11 is also 21 π΄ 22 obtained. Finally, a numerical example is presented to verify the theoretical findings. 1. Introduction The problem of completing a block-partitioned matrix of a specified type with some of its blocks given has been studied by many authors. Fiedler and Markham [1] considered the following completion problem over the real number field R. Suppose π1 , π2 , π1 , π2 are nonnegative integers, π1 + π2 = π1 +π2 = π > 0, π΄ 11 ∈ Rπ1 ×π1 , π΄ 12 ∈ Rπ1 ×π2 , π΄ 21 ∈ Rπ2 ×π1 , and π΅22 ∈ Rπ2 ×π2 . Determine a matrix π΄ 22 ∈ Rπ2 ×π2 such that π΄ π΄ π΄ = ( 11 12 ) π΄ 21 π΄ 22 (1) is nonsingular and π΅22 is the lower right block of a partitioning of π΄−1 . This problem has the form of −1 π΄ π΄ ( 11 12 ) π΄ 21 ? =( ? ? ), ? π΅22 (2) and the solution and the expression for π΄ 22 were obtained in [1]. Dai [2] considered this form of completion problems with symmetric and symmetric positive definite matrices over R. Some other particular forms for 2 × 2 block matrices over R have also been examined (see, e.g., [3]), such as −1 π΅ ? = ( 11 ) , ? ? −1 ? ? = ( ), ? π΅22 π΄ π΄ ( 11 12 ) π΄ 21 ? π΄ ? ( 11 ) ? ? −1 ? π΄ ( 11 ) ? π΄ 22 (3) ? π΅12 = ( ). π΅21 ? The real quaternion matrices play a role in computer science, quantum physics, and so on (e.g., [4–6]). Quaternion matrices are receiving much attention as witnessed recently (e.g., [7–9]). Motivated by the work of [1, 10] and keeping such applications of quaternion matrices in view, in this paper we consider the following completion problem over the real quaternion algebra: H = {π0 + π1 π + π2 π + π3 π | π2 = π2 = π2 = πππ = −1 and π0 , π1 , π2 , π3 ∈ R} . (4) Problem 1. Suppose π1 , π2 , π1 , π2 are nonnegative integers, π1 + π2 = π1 + π2 = π > 0, and π΄ 12 ∈ Hπ1 ×π2 , 2 Journal of Applied Mathematics π΄ 21 ∈ Hπ2 ×π1 , π΄ 22 ∈ π π2 ×π2 , π΅11 ∈ Hπ1 ×π1 . Find a matrix π΄ 11 ∈ Hπ1 ×π1 such that π΄ π΄ π΄ = ( 11 12 ) ∈ Hπ×π π΄ 21 π΄ 22 −1 π΅ ? = ( 11 ) , ? ? π΅ π΅ ( 22 21 ) , π΅12 π΅11 (5) is nonsingular, and π΅11 is the upper left block of a partitioning of π΄−1 . That is ? π΄ 12 ) ( π΄ 21 π΄ 22 Proof. It is readily seen that (6) where Hπ×π denotes the set of all π × π matrices over H and π΄−1 denotes the inverse matrix of π΄. Throughout, over the real quaternion algebra H, we denote the identity matrix with the appropriate size by πΌ, the transpose of π΄ by π΄π , the rank of π΄ by π(π΄), the conjugate transpose of π΄ by π΄∗ = (π΄)π , a reflexive inverse of a matrix π΄ over H by π΄+ which satisfies simultaneously π΄π΄+ π΄ = π΄ and π΄+ π΄π΄+ = π΄+ . Moreover, πΏ π΄ = πΌ−π΄+ π΄, π π΄ = πΌ−π΄π΄+ , where π΄+ is an arbitrary but fixed reflexive inverse of π΄. Clearly, πΏ π΄ and π π΄ are idempotent, and each is a reflexive inverse of itself. R(π΄) denotes the right column space of the matrix π΄. The rest of this paper is organized as follows. In Section 2, we establish some necessary and sufficient conditions to solve Problem 1 over H, and the general expression for π΄ 11 is also obtained. In Section 3, we present a numerical example to illustrate the developed theory. π΄ π΄ ( 22 21 ) π΄ 12 π΄ 11 are inverse to each other, so we may suppose that π(π΄ 11 ) < π(π΅22 ). If π(π΅22 ) = 0, necessarily π(π΄ 11 ) = 0 and we are finished. Let π(π΅22 ) = π > 0, then there exists a matrix πΉ with π right linearly independent columns, such that π΅22 πΉ = 0. Then, using Lemma 1 (singular-value decomposition [9]). Let π΄ ∈ Hπ×π be of rank π. Then there exist unitary quaternion matrices π ∈ Hπ×π and π ∈ Hπ×π such that (7) where π·π = diag(π1 , . . . , ππ ) and the ππ ’s are the positive singular values of π΄. Let Hππ denote the collection of column vectors with π components of quaternions and π΄ be an π × π quaternion matrix. Then the solutions of π΄π₯ = 0 form a subspace of Hππ of dimension π(π΄). We have the following lemma. Lemma 2. Let (8) be a partitioning of a nonsingular matrix π΄ ∈ Hπ×π , and let π΅ π΅ ( 11 12 ) π΅21 π΅22 (11) π΄ 11 π΅12 πΉ = 0. (12) π΄ 21 π΅12 + π΄ 22 π΅22 = πΌ, (13) π΄ 21 π΅12 πΉ = πΉ. (14) From we have It follows that the rank π(π΅12 πΉ) ≥ π. In view of (12), this implies π (π΄ 11 ) ≥ π (π΅12 πΉ) ≥ π = π (π΅22 ) . (15) π (π΄ 11 ) = π (π΅22 ) . (16) Thus In this section, we begin with the following lemmas. π΄ π΄ ( 11 12 ) π΄ 21 π΄ 22 π΄ 11 π΅12 + π΄ 12 π΅22 = 0, we have 2. Main Results π· 0 ππ΄π = ( π ) , 0 0 (10) (9) be the corresponding (i.e., transpose) partitioning of π΄−1 . Then π(π΄ 11 ) = π(π΅22 ). Lemma 3 (see [10]). Let π΄ ∈ Hπ×π , π΅ ∈ Hπ×π , π· ∈ Hπ×π be known and π ∈ Hπ×π unknown. Then the matrix equation π΄ππ΅ = π· (17) π΄π΄+ π·π΅+ π΅ = π·. (18) is consistent if and only if In that case, the general solution is π = π΄+ π·π΅+ + πΏ π΄π1 + π2 π π΅ , (19) where π1 , π2 are any matrices with compatible dimensions over H. By Lemma 1, let the singular value decomposition of the matrix π΄ 22 and π΅11 in Problem 1 be π΄ 22 = π ( Λ 0 ∗ )π , 0 0 (20) Σ 0 ∗ π΅11 = π ( )π , 0 0 (21) Journal of Applied Mathematics 3 where Λ = diag(π 1 , π 2 , . . . , π π ) is a positive diagonal matrix, π π =ΜΈ 0 (π = 1, . . . , π ) are the singular values of π΄ 22 , π = π(π΄ 22 ), Σ = diag(π1 , π2 , . . . , ππ ) is a positive diagonal matrix, ππ =ΜΈ 0 (π = 1, . . . , π) are the singular values of π΅11 and π = π(π΅11 ). π = (π1 π2 ) ∈ Hπ2 ×π2 , π = (π 1 π 2 ) ∈ Hπ2 ×π2 , π = (π1 π2 ) ∈ Hπ1 ×π1 , π = (π1 π2 ) ∈ Hπ1 ×π1 are unitary quaternion matrices, where π1 ∈ Hπ2 ×π , π 1 ∈ Hπ2 ×π , π1 ∈ Hπ1 ×π , and π1 ∈ Hπ1 ×π . It follows that Σ 0 ∗ Λ 0 ∗ ) π π = π∗ π ( ) π πΎπ. π∗ π΄ 21 π ( 0 0 0 0 This implies that ( ∗ (d) R(π΄∗12 π΅11 ) ⊂ R(π΄∗22 ). × (30) Μ=( πΎ Λ−1 π1∗ π΄ 21 π1 Σ 0 + π΄ 12 π ( −1 ) π» −(π2∗ π΄ 12 π 2 ) (22) +π− + ππ΅11 π΅11 , Λ−1 π1∗ π΄ 21 π1 Σ 0 π» πΎ22 ), (32) π» ∈ H(π2 −π )×π , πΎ22 ∈ H(π2 −π )×(π1 −π) . In the same way, from (d), we can obtain where π» is an arbitrary matrix in H arbitrary matrix in Hπ1 ×π1 . (π2 −π )×π and π is an Proof. If there exists an π1 × π1 matrix π΄ 11 such that π΄ is nonsingular and π΅11 is the corresponding block of π΄−1 , then (a) is satisfied. From π΄π΅ = π΅π΄ = πΌ, we have that π΄ 21 π΅11 + π΄ 22 π΅21 = 0, π΅11 π΄ 12 + π΅12 π΄ 22 = 0, π1∗ π΄ 12 π 2 = 0. (33) 12 Notice that ( π΄ π΄ 22 ) in (a) is a full column rank matrix. By (20), (21), and (33), we have Λ 0 0 0 0 π π΄ ( ∗ ) ( 12 ) π = (π∗ π΄ π π∗ π΄ π ) , π 0 π΄ 22 12 1 12 2 1 1 ∗ ∗ π2 π΄ 12 π 1 π2 π΄ 12 π 2 ∗ (23) so that (c) and (d) are satisfied. By (11), we have π (π΄ 22 ) + π (π΄ 22 ) = π2 , (31) Μ From (29), (30), we have Let π ∗ πΎπ = πΎ. In that case, the general solution has the form of + π∗ π΅11 Σ 0 )( ) 0 0 π2∗ π΄ 21 π1 = 0. (c) R(π΄ 21 π΅11 ) ⊂ R(π΄ 22 ), π΄ 11 = π2∗ π΄ 21 π1 π2∗ π΄ 21 π2 Comparing corresponding blocks in (30), we obtain (b) π2 − π(π΄ 22 ) = π1 − π(π΅11 ), that is π2 − π = π1 − π, + π΅11 π1∗ π΄ 21 π1 π1∗ π΄ 21 π2 ∗ ∗ Λ 0 π 1 πΎπ1 π 1 πΎπ2 ). =( )( ∗ 0 0 π πΎπ1 π ∗ πΎπ2 2 2 Theorem 4. Problem 1 has a solution if and only if the following conditions are satisfied: 12 (a) π ( π΄ π΄ 22 ) = π2 , (29) (34) so that π΄ 0 π∗ π΄ 12 ) π ) π2 = π ( 12 ) = π (( ∗ )( π΄ 22 π 0 π΄ 22 π (π΅11 ) + π (π΅11 ) = π1 . (24) 11 From Lemma 2, Notice that ( π΄ π΄ 21 −1 partitioning of π΅ , we have π΄ 12 π΄ 22 ) is the corresponding π (π΅11 ) = π (π΄ 22 ) , Λ 0 0 0 = π (π ∗ π΄ π π ∗ π΄ π ) 12 1 12 2 1 1 (25) π2∗ π΄ 12 π 1 implying that (b) is satisfied. Conversely, from (c), we know that there exists a matrix πΎ ∈ Hπ2 ×π1 such that π΄ 21 π΅11 = π΄ 22 πΎ. (26) π΅21 = −πΎ. (27) Let From (20), (21), and (26), we have Σ 0 ∗ Λ 0 ∗ π΄ 21 π ( )π = π( ) π πΎ. 0 0 0 0 (35) π2∗ π΄ 12 π 2 = π (Λ) + π (π2∗ π΄ 12 π 2 ) = π + π (π2∗ π΄ 12 π 2 ) . It follows from (b) and (35) that π2π π΄ 12 π 2 is a full column rank matrix, so it is nonsingular. From π΄π΅ = πΌ, we have the following matrix equation: π΄ 11 π΅11 + π΄ 12 π΅21 = πΌ, (36) that is (28) π΄ 11 π΅11 = πΌ − π΄ 12 π΅21 , πΌ ∈ Hπ1 ×π1 , (37) 4 Journal of Applied Mathematics where π΅11 , π΄ 12 were given, π΅21 = −πΎ (from (27)). By Lemma 3, the matrix equation (37) has a solution if and only if + π΅11 = πΌ − π΄ 12 π΅21 . (πΌ − π΄ 12 π΅21 ) π΅11 (38) By (21), (27), (32), and (33), we have that (38) is equivalent to: 3. An Example In this section, we give a numerical example to illustrate the theoretical results. Example 5. Consider Problem 1 with the parameter matrices as follows: Σ 0 ∗ Σ−1 0 (πΌ + π΄ 12 πΎ) π ( ) π∗ π ( ) π = πΌ + π΄ 12 πΎ. (39) 0 0 0 0 2+π π΄ 12 = ( We simplify the equation above. The left hand side reduces to (πΌ + π΄ 12 πΎ)π1 π1∗ and so we have π΄ 12 πΎπ1 π1∗ − π΄ 12 πΎ = πΌ − π1 π1∗ . (40) π΄ 21 So, ∗ Μ ∗ π1 π∗ − π΄ 12 π πΎπ Μ ∗ = (π1 π2 ) (π1∗ ) − π1 π∗ . π΄ 12 π πΎπ 1 1 π2 (41) This implies that ∗ ∗ Μ (π1∗ π1 ) π∗ − π΄ 12 π πΎ Μ (π1∗ ) = π2 π∗ , π΄ 12 π πΎ 1 2 π2 π1 π2 3 1 1 1 + π − π− π 2 2 2 2 =( ), 1 1 3 1 π+ π + π 2 2 2 2 2 π π΄ 22 = ( ), 2π π (49) 1 π π΅11 = ( ). π π It is easy to show that (c), (d) are satisfied, and that (42) π΄ π2 = π ( 12 ) = 2, π΄ 22 so that ∗ Μ (πΌ) π∗ − π΄ 12 π πΎ Μ (π1∗ ) = π2 π∗ . π΄ 12 π πΎ 1 2 π2 0 −π 1 π 2 ), 1 1+ π 2 (50) π2 − π (π΄ 22 ) = π1 − π (π΅11 ) = 0, (43) so (a), (b) are satisfied too. Therefore, we have So, Μ ( 0∗ ) = π2 π∗ , −π΄ 12 π πΎ 2 π2 (44) + π΅11 and hence, Λ−1 π1∗ π΄ 21 π1 Σ 0 0 ) ( ∗ ) = π2 π2∗ . − (π΄ 12 π 1 π΄ 12 π 2 ) ( π2 π» πΎ22 (45) 1 1 − π 2 2 =( ), 1 1 − π − π 2 2 Λ 0 ∗ )π , 0 0 π΄ 22 = π ( (51) Σ 0 ∗ π΅11 = π ( )π , 0 0 where Finally, we obtain π΄ 12 π 2 πΎ22 π2∗ = −π2 π2∗ . (46) ∗ Multiplying both sides of (46) by π from the left, considering (33) and the fact that π2∗ π΄ 12 π 2 is nonsingular, we have πΎ22 = −1 −(π2∗ π΄ 12 π 2 ) . π= 1 0 π =( ), 0 1 (47) √2 0 ), Σ=( 0 √2 From Lemma 3, (38), (47), Problem 1 has a solution and the general solution is Λ−1 π1∗ π΄ 21 π1 Σ 0 + + π΄ 12 π ( ) π΄ 11 = π΅11 −1 ∗ π» −(π2 π΄ 12 π 2 ) (48) + + + π − ππ΅11 π΅11 , × π∗ π΅11 where π» is an arbitrary matrix in H(π2 −π )×π and π is an arbitrary matrix in Hπ1 ×π1 . 1 1 π ( ), √2 π π Λ=( π= 2√2 0 ), 0 √2 1 1 π ( ), √2 π π (52) 1 0 π=( ). 0 1 We also have 1 1 π ( ), √2 π π 1 0 π 1 = ( ), 0 1 1 1 π ( ), π1 = √2 π π 1 0 π1 = ( ). 0 1 π1 = (53) Journal of Applied Mathematics 5 By Theorem 4, for an arbitrary matrices π ∈ H2×2 , we have + + + + π΄ 12 π (Λ−1 π1∗ π΄ 21 π1 Σ) π∗ π΅11 + π − ππ΅11 π΅11 π΄ 11 = π΅11 3 1 1 3 1 3 + π+ π + π− π 2 4 4 4 4 2 =( ), 1 1 1 1 3 1 −π+ π− π − π− π−π 2 4 4 4 4 2 (54) it follows that π΄= 3 1 1 3 1 3 1 + π+ π + π− π 2+π π 2 4 4 4 4 2 2 1 1 1 3 1 1 1 −π+ π− π − π − π − π −π 1 + π (2 4 4 4 4 2 2 ) ( ), 1 1 3 1 + π − π− π 2 π 2 2 2 2 1 1 3 1 π+ π + π 2π π ( 2 2 ) 2 2 1 π π΄−1 π π −1 −1 0 −1 1 3 3 − π = (−1 0 ). 4 2 4 1 1 1 1 −1 −1 −π − π− π−π 2 2 2 2 (55) The results verify the theoretical findings of Theorem 4. Acknowledgments The authors would like to give many thanks to the referees and Professor K. C. Sivakumar for their valuable suggestions and comments, which resulted in a great improvement of the paper. This research was supported by Grants from the Key Project of Scientific Research Innovation Foundation of Shanghai Municipal Education Commission (13ZZ080), the National Natural Science Foundation of China (11171205), the Natural Science Foundation of Shanghai (11ZR1412500), the Discipline Project at the corresponding level of Shanghai (A. 13-0101-12-005), and Shanghai Leading Academic Discipline Project (J50101). References [1] M. Fiedler and T. L. Markham, “Completing a matrix when certain entries of its inverse are specified,” Linear Algebra and Its Applications, vol. 74, pp. 225–237, 1986. [2] H. Dai, “Completing a symmetric 2 × 2 block matrix and its inverse,” Linear Algebra and Its Applications, vol. 235, pp. 235– 245, 1996. [3] W. W. Barrett, M. E. Lundquist, C. R. 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