# Partitioned Matrices and the Schur Complement

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1 P Partitioned Matrices and the Schur Complement P 1

2 Appendix P: PARTITIONED MATRICES AND THE SCHUR COMPLEMENT TABLE OF CONTENTS Page P1 Partitioned Matrix P 3 P2 Schur Complements P 3 P3 Block Diagonalization P 3 P4 Determinant Formulas P 3 P5 Partitioned Inversion P 4 P6 Solution of Partitioned Linear System P 4 P7 Rank Additivity P 5 P8 Inertia Additivity P 5 P9 The Quotient Property P 6 P10 Generalized Schur Complements P 6 P Notes and Bibliography P 6 P 2

3 P4 DETERMINANT FORMULAS Partitioned matrices often appear in the exposition of Finite Element Methods This Appendix collects some basic material on the subject Emphasis is placed on results involving the Schur complement This is the name given in the linear algebra literature to matrix objects obtained through the condensation (partial elimination) process discussed in Chapter 10 P1 Partitioned Matrix Suppose that the square matrix M dimensioned (n+m) (n+m), is partitioned into four submatrix blocks as [ ] A B M n n n m (P1) (n+m) (n+m) The dimensions of A, B, C and D are as shown in the block display A and D are square matrices, but B and C are not square unless n m Entries are generally assumed to be complex C m n D m m P2 Schur Complements If A is nonsingular, the Schur complement of M with respect to A is defined as M/A def D CA B (P2) If D is nonsingular, the Schur complement of M with respect to D is defined as M/D def A BD C (P3) Matrices (P2) and (P3) are also called the Schur complement of A in M and the Schur complement of D in M, respectively These equivalent statements are sometimes preferable If both diagonal blocks are singular, see P10 See Notes and Bibliography for other notations in common use P3 Block Diagonalization The following block diagonalization forms, due to Aitken, clearly display the Schur complement role If A is nonsingular, [ ][ ][ I 0 A B I A ] [ ] B A 0, CA I [ ] [ A B C D C D I 0 CA I 0 I ][ A 0 0 M/A 0 M/A ] ][ I A B 0 I (P4) whereas if D is nonsingular [ ][ ][ ] [ ] I BD A B I 0 M/D 0 0 I C D D, C I 0 D [ ] [ ][ ][ ] A B I BD M/D 0 I 0 C D 0 I 0 D D C I (P5) All of these can be directly verified by matrix multiplication P 3

4 Appendix P: PARTITIONED MATRICES AND THE SCHUR COMPLEMENT P4 Determinant Formulas Taking determinants on both sides of the second of (P4) yields det(m) I 0 CA I A 0 0 M/A I A B 0 I A 0 0 M/A det(a) det(m/a) (P6) since the determinant of unit triangular matrices is one Similarly, from the second of (P4) one gets det(m) det(d) det(m/d) (P7) It follows that the determinant is multiplicative in the Schur complement Extensions to the case in which A or D are singular, and its historical connection to Schur s paper [658] are discussed in [817] P5 Partitioned Inversion Suppose that the upper diagonal block A in (P1) is nonsingular Then the inverse of M in partitioned form may be expressed in several ways that involve (P2): [ ] [ M A B I A ][ ][ ] B A 0 I 0 C D 0 I 0 (M/A) CA I [ I A ][ ] B (M/A) A 0 0 (M/A) CA I [ ] [ A 0 A ] (P8) B + (M/A) [ CA I ] 0 0 I [ A + A B (M/A) CA A ] B (M/A) (M/A) CA (M/A) Suppose next that the lower diagonal block D in (P1) is nonsingular Then the form corresponding to the last one above reads [ ] [ M A B (M/D) (M/D) BA ] C D D C (M/D) D + D C (M/D) BD (P9) If both A and D are nonsingular, equating the top left hand block of (P9) to that of the last form of (P8) gives Duncan s inversion formula (M/D) A + A B (M/A) CA (P10) or explicitly in terms of the original blocks, (A BD C) A + A B (D CA B) CA (P11) When n m so that A, B, C and D are all square, and all of them are nonsingular, one obtains the elegant formula due to Aitken: [ ] M (M/D) (M/B) (M/C) (M/A) (P12) Replacing D by the inverse of a matrix, say, T, leads to the Woodbury formula presented in Appendix D If B and C are column and row vectors, respectively, vectors and D a scalar, one obtains the Sherman-Morrison inverse-update formula also presented in that Appendix P 4

5 P8 INERTIA ADDITIVITY P6 Solution of Partitioned Linear System An important use of Schur complements is the partitioned solution of linear systems In fact this is the most important application in FEM, in connection with superelement analysis Suppose that we have the linear system Mz r, in which the given coefficient matrix M is partitioned as in (P1) The RHS r [ pq] T is given, while z [ xy] T is unknown The system is equivalent to the matrix equation pair Ax+ By p, Cx+ Dy q (P13) If D is nonsingular, eliminating y and solving for x yields x (A BD C) (p BD q) (M/D) (p BD q) (P14) Similarly if A is nonsingular, eliminating x and solving for y yields y (D CA B) (q CA p) (M/A) (q CA p) (P15) Readers familiar with the process of static condensation in FEM will notice that (P14) is precisely the process of elimination of internal degrees of freedom of a superelement, as discussed in Chapter 10 In this case M becomes the superelement stiffness matrix K, which is symmetric In the notation of that Chapter, the appropriate block mapping is A K bb, B K bi, C K ib Kbi T, D K ii, p f b, q f i, x u b, and y u i The Schur complement M/D K/K ii is the condensed stiffness matrix K bb K bb K bi K ii force vector f b f b K bi K ii f i K ib, while the RHS of (P14) is the condensed P7 Rank Additivity From (P4) or (P5) we immediately obtain the rank additivity formulas rank(m) rank(a) + rank(m/a) rank(d) + rank(m/d), (P16) or, explicitly in term of the original blocks ([ ]) A B rank rank(a) + rank(d BA C) rank(d) + rank(a CD B), (P17) C D assuming that the indicated inverses exist Consequently the rank is additive in the Schur complement, and so it the inertia of a Hermitian matrix, as considered next P 5

6 Appendix P: PARTITIONED MATRICES AND THE SCHUR COMPLEMENT P8 Inertia Additivity Consider the (generally complex) partitioned Hermitian matrix [ ] A11 A A 12 A (P18) 12 A 22 Here A 11 is assumed nonsingular, and A 12 denotes the conjugate transpose of A 12 The inertia of A is a list of three nonnegative integers In(A) {n +, n, n 0 }, (P19) in which n + n + (A), n n (A), and n 0 n 0 (A) give the number of positive, negative, and zero eigenvalues of A, respectively (Recall that all eigenvalues of a Hermitian matrix are real) Since A is Hermitian, its rank is n + (A) + n (A), and its nullity n 0 (A) (Note that n 0 is the dimension of the kernel of A and also its corank) The inertia additivity formula in terms of the Schur complement of A 11 in A,is In(A) In(A 11 ) + In(A/A 11 ) (P20) It follows that if A 11 is positive definite, and A/A 11 is positive (nonnegative) definite, then A is positive (nonnegative) definite Also A and A/A 11 have the same nullity P9 The Quotient Property Consider the partitioned matrix A [ ] 11 A 12 B1 A B M A C D 21 A 22 B2, C 1 C 2 D (P21) in which both A and A 11 are nonsingular Matrix M need not be square The quotient property of nested Schur complements is M/A (M/A 11 )/(A/A 11 ) (P22) On the RHS of (P22) we see that the denominator matrix A 11 cancels out as if the expressions were scalar fractions If M is square and nonsingular, then taking determinants and aplying the determinant formula we obtain det(m/a) det(m/a 11 )/ det(a/a 11 ) (P23) P10 Generalized Schur Complements If both diagonal blocks A and D of M in (P1) are singular, the notion of Schur complement may be generalized by replacing the conventional inverse with the Moore-Penrose generalized inverse Further developments of this fairly advanced topic may be found in Chapters 1 and 6 of [817] P 6

7 P Notesand Bibliography Notes and Bibliography Most of the following historical remarks are extracted from the Historical Introduction chapter of [817] This edited book gives a comprehensive and up-to-date coverage of the Schur s complement and its use as a basic tool in linear algebra The name Schur complement for the matrices defined in (P2) and (P3) was introduced by Haynsworth in 1968 [345,346] The name was motivated by the seminal determinant lemma by Schur published in 1917 in his studies of stability polynomials [658] Earlier implicit manifestations of this idea starting with Laplace in 1812 are described in the Historical Introduction chapter of [817] Although (P6) and (P7) are now commonly referred to as Schur determinant formulas in the literature, he actually proved only a special case, in which n m so all block matrices in (P1) are square; in addition B and C were required to commute The effect of these assumptions is that the formula can be extended to singular diagonal blocks See 03 of [817] for details The Schur complement appears naturally in the inversion, linear-equation solving, rank and inertia determinations that involve block partitioned matrices Among the most seminal formulas are: The last of the partitioned inversion formulas (P8) was first published in 1937 by Banachievicz [44] It appeared one year later (independently derived) in the classical monograph by Frazer, Duncan and Collar [282], cited in Appendix H as one of the early sources for Matrix Structural Analysis The Duncan inversion formula (P10) appeared in 1944 [192] It is a follow up to the inversion methods of [282] The Aitken block diagonalization formulas (P4) and (P5) appear in another classic book from the same period [7] The inertia additivity formula (P20) as well as the quotient property (P22) are due to Haynsworth and coworkers [345,346]; see also [156,347,533,534] From the Duncan inversion formula follows directly the Guttman rank additivity formula (P16) first published in 1946 [322], as well as the already cited inertia additivity formula for Hermitian matrices (P20), which appeared much later [345,346] The notation M/A for D CA B is now commonly used in linear algebra Others are (M/A), M A, (M A), and M A The latter has visualization problems if matrix blocks are subscripted; for example M A22 is clumsier to read than M/A 22 or M A 22 A drawback of the slash (and vertical bar) notation is that enclosing parentheses are necessary if one takes a transpose or inverse; eg, compare (M/A 22 ) versus the more compact M A 22 P 7

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