# Lecture Notes 1: Matrix Algebra Part B: Determinants and Inverses

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1 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 1 of 57 Lecture Notes 1: Matrix Algebra Part B: Determinants and Inverses Peter J. Hammond Autumn 2012, revised Autumn 2014

2 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 2 of 57 Lecture Outline Determinants Determinants of Order 2 Determinants of Order 3 Characterizing the Determinant Function Rules for Determinants Expansion by Alien Cofactors and the Adjugate Matrix Minor Determinants The Inverse Matrix Definition and Existence Orthogonal Matrices Partitioned Matrices

3 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 3 of 57 Outline Determinants Determinants of Order 2 Determinants of Order 3 Characterizing the Determinant Function Rules for Determinants Expansion by Alien Cofactors and the Adjugate Matrix Minor Determinants The Inverse Matrix Definition and Existence Orthogonal Matrices Partitioned Matrices

4 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 4 of 57 Determinants of Order 2: Definition Consider again the pair of linear equations a 11 x 1 + a 12 x 2 = b 1 a 21 x 1 + a 12 x 2 = b 2 with its associated coefficient matrix ( ) a11 a A = 12 a 21 a 22 Let us define D := a 11 a 22 a 21 a 12. Provided that D 0, there is a unique solution given by x 1 = 1 D (b 1a 22 b 2 a 12 ), x 2 = 1 D (b 2a 11 b 1 a 21 ) The number D is called the determinant of the matrix A, and denoted by either det(a) or more concisely, A.

5 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 5 of 57 Determinants of Order 2: Simple Rule Thus, for any 2 2 matrix A, its determinant D is A = a 11 a 12 a 21 a 22 = a 11a 22 a 21 a 12 For this special case of order 2 determinants, a simple rule is: 1. multiply the diagonal elements together; 2. multiply the off-diagonal elements together; 3. subtract the product of the off-diagonal elements from the product of the diagonal elements. Note that 1 0 A = a 11 a a a

6 Cramer s Rule in the 2 2 Case Using determinant notation, the solution to the equations a 11 x 1 + a 12 x 2 = b 1 a 21 x 1 + a 12 x 2 = b 2 can be written in the alternative form x 1 = 1 D b 1 a 12 b 2 a 22, x 2 = 1 D a 11 b 1 a 21 b 2 This accords with Cramer s rule for the solution to Ax = b, which is the vector x = (x i ) n i=1 each of whose components x i is the fraction with: 1. denominator equal to the determinant D of the coefficient matrix A (provided, of course, that D 0); 2. numerator equal to the determinant of the matrix (A i, b) formed from A by replacing its ith column with the b vector of right-hand side elements. University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 6 of 57

7 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 7 of 57 Outline Determinants Determinants of Order 2 Determinants of Order 3 Characterizing the Determinant Function Rules for Determinants Expansion by Alien Cofactors and the Adjugate Matrix Minor Determinants The Inverse Matrix Definition and Existence Orthogonal Matrices Partitioned Matrices

8 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 8 of 57 Determinants of Order 3: Definition Determinants of order 3 can be calculated from those of order 2 according to the formula a A = a 22 a a 32 a 33 a 12 a 21 a 23 a 31 a 33 + a 13 a 21 a 22 a 31 a 32 = 3 j=1 ( 1)1+j a 1j C 1j where, for j = 1, 2, 3, the 2 2 matrix C 1j is the (1, j)-cofactor obtained by removing both row 1 and column j from A. The result is the following sum A = a 11 a 22 a 33 a 11 a 23 a 32 + a 12 a 23 a 31 a 12 a 21 a 33 + a 13 a 21 a 32 a 13 a 22 a 31 of 3! = 6 terms, each the product of 3 elements chosen so that each row and each column is represented just once.

9 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 9 of 57 Determinants of Order 3: Cofactor Expansion The determinant expansion A = a 11 a 22 a 33 a 11 a 23 a 32 + a 12 a 23 a 31 a 12 a 21 a 33 + a 13 a 21 a 32 a 13 a 22 a 31 is very symmetric, suggesting (correctly) that the cofactor expansion along the first row (a 11, a 12, a 13 ) A = 3 j=1 ( 1)1+j a 1j C 1j gives the same answer as the two cofactor expansions A = 3 along, respectively: j=1 ( 1)r+j a rj C rj = 3 the rth row (a r1, a r2, a r3 ) the sth column (a 1s, a 2s, a 3s ) i=1 ( 1)i+s a is C is

10 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 10 of 57 Determinants of Order 3: Alternative Expressions The same result A = a 11 a 22 a 33 a 11 a 23 a 32 + a 12 a 23 a 31 a 12 a 21 a 33 + a 13 a 21 a 32 a 13 a 22 a 31 can be obtained as either of the two expansions 3 3 A = 3 ɛ j j 1 =1 j 2 =1 j 3 =1 1 j 2 j 3 a 1j1 a 2j2 a 3j3 = sgn(π) 3 π Π i=1 a iπ(i) Here ɛ j = ɛ j1 j 2 j 3 { 1, 0, 1} denotes the Levi-Civita symbol associated with the mapping i j i from {1, 2, 3} into itself. Also, Π denotes the set of all 3! = 6 possible permutations on {1, 2, 3}, with typical member π, whose sign is denoted by sgn(π).

11 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 11 of 57 Outline Determinants Determinants of Order 2 Determinants of Order 3 Characterizing the Determinant Function Rules for Determinants Expansion by Alien Cofactors and the Adjugate Matrix Minor Determinants The Inverse Matrix Definition and Existence Orthogonal Matrices Partitioned Matrices

12 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 12 of 57 The Determinant Function When n = 1, 2, 3, the determinant mapping A A R specifies the determinant A of each n n matrix A as a function of its n row vectors (a i ) n i=1. For a general natural number n N, consider any mapping D n A D(A) = D ((a i ) n i=1) R defined on the domain D n of n n matrices. Notation: Let D(A/b r ) denote the new value D(a 1,..., a r 1, b r, a r+1,..., a n ) of the function D after the rth row a r of the matrix A has been replaced by the new row vector b r.

13 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 13 of 57 Row Multilinearity Definition The function D n A D(A) of A s n rows (a i ) n i=1 is (row) multilinear just in case, for each row number i {1, 2,..., n}, each pair b i, c i R n of new versions of row i, and each pair of scalars λ, µ R, one has D(A/λb i + µc i ) = λd(a/b i ) + µd(a/c i ) Formally, the mapping R n a i D(A/a i ) R should be linear, for fixed each row i N n. That is, D is a linear function of the ith row vector a i on its own, when all the other rows a h (h i) are fixed.

14 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 14 of 57 The Three Characterizing Properties Definition The function D n A D(A) is alternating just in case for every transposition matrix T, one has D(TA) = D(A) i.e., interchanging any two rows reverses its sign. Definition The mapping D n A D(A) is of the determinant type just in case: 1. D is multilinear in its rows; 2. D is alternating; 3. D(I n ) = 1 for the identity matrix I n. Exercise Show that the mapping D n A A R is of the determinant type provided that n 3.

15 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 15 of 57 First Implication of Multilinearity in the n n Case Lemma Suppose that D n A D(A) is multilinear in its rows. For any fixed B D n, the value of D(AB) can be expressed as the linear combination D(AB) = n n... j 1 =1 j 2 =1 n a 1j1 a 2j2 a njn D(L j1 j 2...j n B) j n=1 of its values at all possible matrices L j B = L j1 j 2...j n B := (b jr ) n r=1 whose rth row, for each r = 1, 2,..., n, equals the j r th row b jr of the matrix B.

16 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 16 of 57 Characterizing 2 2 Determinants 1. In the case of 2 2 matrices, the lemma tells us that multilinearity implies D(AB) = a 11 a 21 D(b 1, b 1 ) + a 11 a 22 D(b 1, b 2 ) + a 12 a 21 D(b 2, b 1 ) + a 12 a 22 D(b 2, b 2 ) where b 1 = (b 11, b 21 ) and b 2 = (b 12, b 22 ) are the rows of B. 2. If D is also alternating, then D(b 1, b 1 ) = D(b 2, b 2 ) = 0 and D(B) = D(b 1, b 2 ) = D(b 2, b 1 ), implying that D(AB) = a 11 a 22 D(b 1, b 2 ) + a 12 a 21 D(b 2, b 1 ) = (a 11 a 22 a 12 a 21 )D(B) 3. Imposing the additional restriction D(B) = 1 when B = I 2, we obtain the ordinary determinant D(A) = a 11 a 22 a 12 a Then, too, one derives the product rule D(AB) = D(A)D(B).

17 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 17 of 57 First Implication of Multilinearity: Proof Each element of the product C = AB satisfies c ik = n j=1 a ijb jk. Hence each row c i = (c ik ) n k=1 of C can be expressed as the linear combination c i = n j=1 a ijb j of B s rows. For each r = 1, 2,..., n and arbitrary selection b j1,..., b jr 1 of r 1 rows from B, multilinearity therefore implies that D(b j1,..., b jr 1, c r, c r+1,..., c n ) = n j r =1 a ij r D(b j1,..., b jr 1, b jr, c r+1,..., c n ) This equation can be used to show, by induction on k, that D(C) = n n... j 1 =1 j 2 =1 n a 1j1 a 2j2 a kjk D(b j1,..., b jk, c k+1,..., c n ) j k =1 for k = 1, 2,..., n, including for k = n as the lemma claims.

18 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 18 of 57 Additional Implications of Alternation Lemma Suppose D n A D(A) is both row multilinear and alternating. Then for all possible n n matrices A, B, and for all possible permutation matrices P π, one has: 1. D(AB) = π Π n i=1 a iπ(i)d(p π B) 2. D(P π B) = sgn(π)d(b). 3. Under the additional assumption that D(I n ) = 1, one has: determinant formula: D(A) = π Π sgn(π) n i=1 a iπ(i); product rule: D(AB) = D(A)D(B)

19 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 19 of 57 First Additional Implication of Alternation: Proof Because D is alternating, one has D(B) = 0 whenever two rows of B are equal. It follows that for any matrix (b ji ) n i=1 = L jb whose n rows are all rows of the matrix B, one has D((b ji ) n i=1 ) = 0 unless these rows are all different. But if all the n rows of (b ji ) n i=1 = L jb are different, there exists a permutation π Π such that L j B = P π B. Hence, after eliminating terms that are zero, the sum D(AB) = n n j 1 =1 j 2 =1... n j a n=1 1j 1 a 2j2 a njn D((b jr ) n r=1 ) = n n j 1 =1 j 2 =1... n j a n=1 1j 1 a 2j2 a njn D(L j1 j 2...j n B) as stated in part 1 of the Lemma.

20 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 20 of 57 Second Additional Implication: Proof Because D is alternating, one has D(TP π B) = D(P π B) whenever T is a transposition matrix. Suppose that π = τ 1 τ q is one possible factorization of the permutation π as the composition of transpositions. But sgn(τ) = 1 for any transposition τ. So sgn(π) = ( 1) q by the product rule for signs of permutations. Note that P π = T 1 T 2 T q where T p denotes the permutation matrix corresponding to the transposition τ p, for each p = 1,..., q It follows that D(P π B) = D(T 1 T 2 T q B) = ( 1) q D(B) = sgn(π)d(b) as required.

21 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 21 of 57 Third Additional Implication: Proof In case D(I n ) = 1, applying parts 1 and 2 of the Lemma (which we have already proved) with B = I n gives immediately D(A) = π Π n i=1 a iπ(i)d(p π ) = π Π sgn(π) n But then, applying parts 1 and 2 of the Lemma for a general matrix B gives n i=1 a iπ(i) D(AB) = a iπ(i)d(p π B) π Π i=1 = sgn(π) n a iπ(i)d(b) = D(A)D(B) π Π i=1 as an implication of the first equality on this slide. This completes the proof of all three parts.

22 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 22 of 57 Formal Definition and Cofactor Expansion Definition The determinant A of any n n matrix A is defined so that D n A A is the unique (row) multilinear and alternating mapping that satisfies I n = 1. Definition For any n n determinant A, its rs-cofactor C rs is the (n 1) (n 1) determinant of the matrix C rs obtained by omitting row r and column s from A. The cofactor expansion of A along any row r or column s is A = n j=1 ( 1)r+j a rj C rj = n i=1 ( 1)i+s a is C is Exercise Prove that these cofactor expansions are valid, using the formula A = π Π n i=1 sgn(π)a iπ(i)

23 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 23 of 57 Outline Determinants Determinants of Order 2 Determinants of Order 3 Characterizing the Determinant Function Rules for Determinants Expansion by Alien Cofactors and the Adjugate Matrix Minor Determinants The Inverse Matrix Definition and Existence Orthogonal Matrices Partitioned Matrices

24 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 24 of 57 Eight Basic Rules (Rules A H of EMEA, Section 16.4) Let A denote the determinant of any n n matrix A. 1. A = 0 if all the elements in a row (or column) of A are A = A, where A is the transpose of A. 3. If all the elements in a single row (or column) of A are multiplied by a scalar α, so is its determinant. 4. If two rows (or two columns) of A are interchanged, the determinant changes sign, but not its absolute value. 5. If two of the rows (or columns) of A are proportional, then A = The value of the determinant of A is unchanged if any multiple of one row (or one column) is added to a different row (or column) of A. 7. The determinant of the product AB of two n n matrices equals the product A B of their determinants. 8. If α is any scalar, then αa = α n A.

25 The Transpose Rule 2: Verification The transpose rule 2 is key: for any statement about how A depends on the rows of A, there is an equivalent statement about how A depends on the columns of A. Exercise Verify Rule 2 directly for 2 2 and then for 3 3 matrices. Proof of Rule 2 The expansion formula implies that A = π Π sgn(π) n i=1 a iπ(i) = π Π sgn(π) n But the product rule for signs of permutations implies that sgn(π) sgn(π 1 ) = sgn(ι) = 1, with sgn(π) = ±1. Hence sgn(π 1 ) = 1/ sgn(π) = sgn(π). So, because π π 1 is a bijection, A = π 1 Π sgn(π 1 ) n j=1 a jπ 1 (j) = A after using the expansion formula with π replaced by π 1. j=1 a π 1 (j)j University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 25 of 57

26 Verification of Rule 6 Exercise Verify Rule 6 directly for 2 2 and then for 3 3 matrices. Proof of Rule 6 Recall the notation E r+αq for the matrix resulting from adding the multiple of α times row q of I to its rth row. Recall too that E r+αq A is the matrix that results from applying the same row operation to the matrix A. Finally, recall the formula A = n j=1 a rj C rj for the cofactor expansion of A along the rth row. The corresponding cofactor expansion of E r+αq A is then E r+αq A = n j=1 (a rj + αa qj ) C rj = A + α B where B is derived from A by replacing row r with row q. Unless q = r, the matrix B will have its qth row repeated, implying that B = 0 because the determinant is alternating. So q r implies E r+αq A = A for all α, which is Rule 6. University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 26 of 57

27 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 27 of 57 Verification of the Other Rules Apart from Rules 2 and 6, note that we have already proved the product Rule 7, whereas the interchange Rule 4 just restates alternation. Now that we have proved Rule 2, note that Rules 1 and 3 follow from multilinearity, applied in the special case when one row of the matrix is multiplied by a scalar. Also, the proportionality Rule 5 follows from combining Rule 4 with multilinearity. Finally, Rule 8, concerning the effect of multiplying all elements of a matrix by the same scalar, is easily checked because the expansion of A is the sum of many terms, each of which involves the product of exactly n elements of A.

28 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 28 of 57 Outline Determinants Determinants of Order 2 Determinants of Order 3 Characterizing the Determinant Function Rules for Determinants Expansion by Alien Cofactors and the Adjugate Matrix Minor Determinants The Inverse Matrix Definition and Existence Orthogonal Matrices Partitioned Matrices

29 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 29 of 57 Expansion by Alien Cofactors Expanding along either row r or column s gives A = n j=1 a rj C rj = n when one uses matching cofactors. Expanding by alien cofactors, however, from either the wrong row i r or the wrong column j s, gives 0 = n j=1 a rj C ij = n i=1 a is C is i=1 a is C ij This is because the answer will be the determinant of an alternative matrix in which: either row i has been duplicated and put in row r; or column j has been duplicated and put in column s.

30 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 30 of 57 The Adjugate Matrix Definition The adjugate (or (classical) adjoint ) adj A of an order n square matrix A has elements given by (adj A) ij = C ji. It is therefore the transpose of the cofactor matrix C + whose elements are the respective cofactors of A.

31 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 31 of 57 Main Property of the Adjugate Matrix Theorem (adj A)A = A(adj A) = A I n for every n n square matrix A. Proof. The (i, j) elements of the two product matrices are [(adj A)A] ij = n These are expansions by: k=1 C ki a kj and [A(adj A)] ij = n k=1 a ik C jk alien cofactors in case i j, implying that they equal 0; matching cofactors in case i = j, implying that they equal A. Hence [(adj A)A] ij = [A(adj A)] ij = A (I n ) ij for each pair (i, j).

32 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 32 of 57 Outline Determinants Determinants of Order 2 Determinants of Order 3 Characterizing the Determinant Function Rules for Determinants Expansion by Alien Cofactors and the Adjugate Matrix Minor Determinants The Inverse Matrix Definition and Existence Orthogonal Matrices Partitioned Matrices

33 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 33 of 57 Minor Determinants: Definition Definition Given any m n matrix A, a minor determinant of order k is the determinant A i1 i 2...i k,j 1 j 2...j k of a k k submatrix (a ij ), with 1 i 1 < i 2 <... < i k m and 1 j 1 < j 2 <... < j k n, that is formed by selecting all the elements that lie both: in one of the chosen rows i r (r = 1, 2,..., k); in one of the chosen columns j s (s = 1, 2,..., k). Example 1. In case A is an n n matrix: the whole determinant A is the only minor of order n; each of the n 2 cofactors C ij is a minor of order n 1; 2. In case A is an m n matrix: each element of the mn elements of the matrix is a minor( of order 1; there are m ) ( k n ) k minors of order k.

34 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 34 of 57 Principal and Leading Principal Minors Exercise Verify that the set of elements that make up the minor A i1 i 2...i k,j 1 j 2...j k of order k is completely determined by its k diagonal elements a ih,j h (h = 1, 2,..., k). Definition If A is an n n matrix, the minor A i1 i 2...i k,j 1 j 2...j k of order k is: a principal minor if all its diagonal elements are diagonal elements of A; a leading principal minor if its diagonal elements are a hh (h = 1, 2,..., k). Exercise Explain why an n n determinant has 2 n 1 principal minors.

35 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 35 of 57 Outline Determinants Determinants of Order 2 Determinants of Order 3 Characterizing the Determinant Function Rules for Determinants Expansion by Alien Cofactors and the Adjugate Matrix Minor Determinants The Inverse Matrix Definition and Existence Orthogonal Matrices Partitioned Matrices

36 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 36 of 57 Definition of Inverse Matrix Exercise Suppose that A is any invertible n n matrix for which there exist n n matrices B and C such that AB = CA = I. 1. By writing CAB in two different ways, prove that B = C. 2. Use this result to show that the equal matrices B = C, if they exist, must be unique. Definition The n n matrix X is the unique inverse of the invertible n n matrix A provided that AX = XA = I n. In this case we write X = A 1, so A 1 denotes the unique inverse. Big question: does the inverse exist?

37 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 37 of 57 Existence Conditions Theorem An n n matrix A has an inverse if and only if A 0, which holds if and only if at least one of the equations AX = I n and XA = I n has a solution. Proof. Provided A 0, the identity (adj A)A = A(adj A) = A I n shows that the matrix X := (1/ A ) adj A is well defined and satisfies XA = AX = I n, so X is the inverse A 1. Conversely, if either XA = I n or AX = I n has a solution, then the product rule for determinants implies that 1 = I n = AX = XA = A X, and so A 0. The rest follows from the paragraph above.

38 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 38 of 57 Singularity So A 1 exists if and only if A 0. Definition 1. In case A = 0, the matrix A is said to be singular; 2. In case A 0, the matrix A is said to be non-singular or invertible.

39 Example and Application to Simultaneous Equations Exercise Verify that A = ( ) = A 1 = C := ( by using direct multiplication to show that AC = CA = I 2. Example Suppose that a system of n simultaneous equations in n unknowns is expressed in matrix notation as Ax = b. Of course, A must be an n n matrix. Suppose A has an inverse A 1. Premultiplying both sides of the equation Ax = b by this inverse gives A 1 Ax = A 1 b, which simplifies to Ix = A 1 b. Hence the unique solution of the equation is x = A 1 b. University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 39 of 57 )

40 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 40 of 57 Cramer s Rule: Statement Notation Given any m n matrix A, let [A j, b] denote the new m n matrix in which column j has been replaced by the column vector b. Evidently [A j, a j ] = A. Theorem Provided that the n n matrix A is invertible, the simultaneous equation system Ax = b has a unique solution x = A 1 b whose ith component is given by the ratio of determinants x i = [A i, b] / A. This result is known as Cramer s rule.

41 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 41 of 57 Cramer s Rule: Proof Proof. Given the equation system AX = b, each cofactor C ji of the coefficient matrix A is also the (j, i) cofactor of the matrix [A i, b]. Expanding the determinant [A i, b] by cofactors along column i therefore gives n j=1 b j C ji = n j=1 (adj A) ijb j, by definition of the adjugate matrix. Hence the unique solution to the equation system has components x i = (A 1 b) i = 1 n A (adj A) ijb j = 1 j=1 A [A i, b] for i = 1, 2,..., n.

42 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 42 of 57 Rule for Inverting Products Theorem Suppose that A and B are two invertible n n matrices. Then the inverse of the matrix product AB exists, and is the reverse product B 1 A 1 of the inverses. Proof. Using the associative law for matrix multiplication repeatedly gives: (B 1 A 1 )(AB) = B 1 (A 1 A)B = B 1 (I)B = B 1 (IB) = B 1 B = I and (AB)(B 1 A 1 ) = A(BB 1 )A 1 = A(I)A 1 = (AI)A 1 = AA 1 = I. These equations confirm that X := B 1 A 1 is the unique matrix satisfying the double equality (AB)X = X(AB) = I.

43 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 43 of 57 Rule for Inverting Chain Products Exercise Prove that, if A, B and C are three invertible n n matrices, then (ABC) 1 = C 1 B 1 A 1. Then use mathematical induction to extend this result in order to find the inverse of the product A 1 A 2 A k of any finite chain of invertible n n matrices.

44 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 44 of 57 Matrices for Elementary Row Operations Example Consider the following two out of the three possible kinds of elementary row operation: 1. of multiplying the rth row by α R, represented by the matrix S r (α); 2. of multiplying the qth row by α R, then adding the result to row r, represented by the matrix E r+αq. Exercise Find the determinants and, when they exist, the inverses of the matrices S r (α) and E r+αq.

45 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 45 of 57 Outline Determinants Determinants of Order 2 Determinants of Order 3 Characterizing the Determinant Function Rules for Determinants Expansion by Alien Cofactors and the Adjugate Matrix Minor Determinants The Inverse Matrix Definition and Existence Orthogonal Matrices Partitioned Matrices

46 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 46 of 57 Inverting Orthogonal Matrices An n-dimensional square matrix Q is said to be orthogonal just in case its columns form an orthonormal set i.e., they must be pairwise orthogonal unit vectors. Theorem A square matrix Q is orthogonal if and only if it satisfies Q Q = I. Proof. The elements of the matrix product Q Q satisfy (Q Q) ij = n k=1 q ik q kj = n k=1 q kiq kj = q i q j where q i (resp. q j ) denotes the ith (resp. jth) column vector of Q. But the columns of Q are orthonormal iff q i q j = δ ij for all i, j = 1, 2,..., n, and so iff Q Q = I.

47 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 47 of 57 Exercises on Orthogonal Matrices Exercise Show that if the matrix Q is orthogonal, then so is Q. Use this result to show that a matrix is orthogonal if and only if its row vectors also form an orthonormal set. Exercise Show that any permutation matrix is orthogonal.

48 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 48 of 57 Rotations in R 2 Example In R 2, consider the anti-clockwise rotation through an angle θ of the unit circle S 1 = {(x 1, x 2 ) R 2 x x 2 2 = 1}. It maps: 1. the first unit vector (1, 0) of the canonical basis to the column vector (cos θ, sin θ) ; 2. the second unit vector (0, 1) of the canonical basis to the column vector ( sin θ, cos θ). So the rotation can be represented by the rotation matrix ( ) cos θ sin θ R θ := sin θ cos θ with these vectors as its columns.

49 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 49 of 57 Rotations in R 2 Are Orthogonal Matrices Because sin( θ) = sin(θ) and cos( θ) = cos(θ), the transpose of R θ satisfies R θ = R θ, and so is the clockwise rotation through an angle θ of the unit circle S 1. Since clockwise and anti-clockwise rotations are inverse operations, it is no surprise that R θ R θ = I. We verify this algebraically by using matrix multiplication ( ) ( ) ( ) R cos θ sin θ cos θ sin θ 1 0 θ R θ = = = I sin θ cos θ sin θ cos θ 0 1 because cos 2 θ + sin 2 θ = 1, thus verifying orthogonality. Similarly ( R θ R cos θ sin θ θ = sin θ cos θ ) ( ) cos θ sin θ = sin θ cos θ ( ) 1 0 = I 0 1

50 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 50 of 57 Outline Determinants Determinants of Order 2 Determinants of Order 3 Characterizing the Determinant Function Rules for Determinants Expansion by Alien Cofactors and the Adjugate Matrix Minor Determinants The Inverse Matrix Definition and Existence Orthogonal Matrices Partitioned Matrices

51 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 51 of 57 Partitioned Matrices: Definition A partitioned matrix is a rectangular array of different matrices. Example Consider the (m + l) (n + k) matrix ( ) A B C D where the four submatrices A, B, C, D are of dimension m n, m k, l n and l k respectively. For any scalar α R, the scalar multiple of a partitioned matrix is ( ) ( ) A B αa αb α = C D αc αd

52 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 52 of 57 Partitioned Matrices: Addition Suppose the two partitioned matrices ( ) A B and C D ( ) E F G H have the property that the following four pairs of corresponding matrices have equal dimensions: (i) A and E; (ii) B and F; (iii) C and G; (iv) D and H. Then the sum of the two matrices is ( ) ( ) ( ) A B E F A + E B + F + = C D G H C + G D + H

53 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 53 of 57 Partitioned Matrices: Multiplication Provided that the two partitioned matrices ( ) ( ) A B E F and C D G H along with their sub-matrices are all compatible for multiplication, the product is defined as ( ) ( ) ( ) A B E F AE + BG AF + BH = C D G H CE + DG CF + DH This adheres to the usual rule for multiplying rows by columns.

54 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 54 of 57 Transposes and Some Special Matrices The rule for transposing a partitioned matrix is C ( A B ) ( A = C ) D B D So the original matrix is symmetric iff A = A, D = D, B = C, and C = B. It is diagonal iff A, D are both diagonal, while B = 0 and C = 0. The identity matrix is diagonal with A = I, D = I, possibly identity matrices of different dimensions.

55 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 55 of 57 Partitioned Matrices: Inverses, I For an (m + n) (m + n) partitioned matrix to have an inverse, the equation ( ) ( ) ( ) ( ) A B E F AE + BG AF + BH Im 0 = = n m C D G H CE + DG CF + DH 0 m n I n should have a solution for the matrices E, F, G, H, given A, B, C, D. Assuming that A has an inverse, we can: 1. construct new first m equations by premultiplying the old ones by A 1 ; 2. construct new second n equations by: premultiplying the new first m equations by C; then subtracting this product from the old second n equations. The result is ( Im A 1 ) ( ) B E F 0 D CA 1 = B G H ( A 1 ) 0 n m CA 1 I n

56 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 56 of 57 Partitioned Matrices: Inverses, II To take the next step, assume the matrix X := D CA 1 B also has an inverse X 1 = (D CA 1 B) 1. ( Im A Given 1 ) ( ) ( B E F A D CA 1 = n m B G H CA 1 I n we can then premultiply the second n equations by X 1, then subtract A 1 B times the new second n equations from the old first m equations to obtain ) ( ) ( ) E F E F ( Im 0 n m = = Z 0 m n I n G H G H ( A where Z := 1 + A 1 BX 1 CA 1 A 1 BX 1 ) X 1 CA 1 X 1 Exercise Use direct multiplication twice in order to verify that ( ) ( ) ( ) A B A B Im 0 Z = Z = n m C D C D 0 m n I n ),

57 University of Warwick, EC9A0 Maths for Economists Peter J. Hammond 57 of 57 Partitioned Matrices: Extension Exercise Suppose that the two partitioned matrices A = (A ij ) k l and B = (B ij ) k l are both k l arrays of respective m i n j matrices A ij, B ij. 1. Under what conditions can the product AB be defined as a k l array of matrices? 2. Under what conditions can the product BA be defined as a k l array of matrices? 3. When either AB or BA can be so defined, give a formula for its product, using summation notation. 4. Express A as a partitioned matrix. 5. Under what conditions is the matrix A symmetric?

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