Lecture 11. Shuanglin Shao. October 2nd and 7th, 2013

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1 Lecture 11 Shuanglin Shao October 2nd and 7th, 2013

2 Matrix determinants: addition. Determinants: multiplication. Adjoint of a matrix. Cramer s rule to solve a linear system.

3 Recall that from the previous section, if a single row of A is multiplied by k, then for the resulting matrix, det(b) = k det(a). If the matrix B = ka, i.e., every entries in A are multiplied by the constant k, then we iterated the previous theorem det(b) = k n det(a).

4 Example: det(a + B) det(a) + det(b). Consider A = [ ] [ 3 1, B = 1 3 ] [ 4 3?A + B == 3 8 We have det(a) = 1, det(b) = 8, and det(a + B) = 23. Thus det(a + B) det(a) + det(b). ].

5 Theorem. Let A, B and C be n n matrices that differ only in a single row, say the r-th row, and assume that the r-th row can be obtained by adding corresponding entries in the r-th row of A and B, then det(c) = det(a) + det(b). This is proved by row expansion of determinants along the r-th row of C. Suppose that... A = a r1 a r2 a rn ; B =... b r1 b r2 b rn, and... C = a r1 + b r1 a r2 + b r2 a rn + b rn,... where the rest entires are the same.

6 We first observe that, if fixing the (r, j)-entry in either A, B or C, then the cofactors are the same, i.e., C rj is identical for A, B and C. det(c) = = n (a rj + b rj )C rj j=1 n a rj C rj + j=1 n b rj C rj j=1 = det(a) + det(b). Note that A + B is not the same as C: A + B is obtained by adding the corresponding entries.

7 Example ( 1) =

8 Determinants of a product. Lemma If B is an n n matrix and E is an n n elementary matrix, then det(eb) = det(e) det(b). Let E be an elementary matrix. If we multiply B by E from the left, EB is the matrix obtained by performing an elementary row operation on B. Case 1. If E is obtained from I n by multiplying k to a row, then EB is the matrix that is obtained by multiplying k to the same row. because det(e) = k. det(eb) = k det(b) = det(e) det(b)

9 Case 2. If E is obtained from I n by exchanging two rows, then EB is the matrix that is obtained by exchanging the same two rows: because det(e) = 1. det(eb) = det(b) = det(e) det(b) Case 3. If E is obtained from I n by adding k times a row to another row, then EB is the matrix that is obtained by adding k times the same row to the same another row: because det(e) = 1. det(eb) = det(b) = det(e) det(b)

10 Remark. If B is an n n matrix and E 1, E 2,, E r are n n elementary matrices, then det(e 1 E 2 E r B) = det(e 1 ) det(e 2 ) det(e r ) det(b).

11 Determinant test for invertibility Theorem. det(a) 0. A square matrix A is invertible if and only if Proof. If A is invertible, then A is expressed as a product of elementary matrices: A = E 1 E 2 E r. Then det(a) = det(e 1 ) det(e 2 ) det(e r ). Since for each elementary matrix E, det(e) 0. Thus det(a) 0.

12 Cont. On the other hand, suppose det(a) 0. We apply elementary row operations to reduce A to reduced row echelon form R. That is to say, R = E 1 E 2 E r A. Then This implies det(r) = det(e 1 ) det(e 2 ) det(e r ) det(a) 0. det(r) 0. Since R is a square matrix and is in reduced row echelon form, R is the identity matrix. In other words, A reduces to I n after a series of elementary row operations. Hence A is invertible.

13 Example. Let A = A is proportional, Hence A is not invertible Since the first and third rows of det(a) = 0.

14 If A and B are square matrices of the same size, then det(ab) = det(a) det(b). Case 1. If A is not invertible, det(a) = 0. It also follows that AB is not invertible, which implies that det(ab) = 0. Thus det(ab) = 0 = det(a) B. Case 2. If A is invertible, then A is a product of elementary matrices, E 1, E 2,, E r : A = E 1 E 2 E r B.

15 Thus det(ab) = det(e 1 E 2 E r B) = det(e 1 ) det(e 2 ) det(e r ) det(b). Since det(a) = det(e 1 ) det(e 2 ) det(e r ), det(ab) = det(a) det(b).

16 [ 3 1 Consider A = 2 1 The product We verify that ] [ 1 3, B = 5 8 AB = [ ]. ]. det(a) = 1, det(b) = 23, det(ab) = 23. Thus det(ab) = det(a) det(b).

17 A corollary to the theorem is Theorem. If A is invertible, then det(a 1 ) = 1 det(a). This is because Thus I n = A A 1. 1 = det(i n ) = det(a) det(a 1 ).

18 Def. If A is any n n matrix, and C ij is the cofactor of a ij, then the matrix C 11 C 21 C n1 C 12 C 22 C n2... C 1n C 2n C nn is called the adjoint of the matrix A, denoted by adj(a). Recall that C ij = ( 1) i+j M ij, and M ij is the determinant of the (n 1) (n 1) matrix obtained from A by removing the i-th row and the j-th column of A.

19 Example. Let The adjoint of A is A = adj(a) = We compute C 21 = ( 1) = 4.

20 Inverse of a matrix using its adjoint. Theorem. If A is an invertible matrix, then A 1 = 1 det(a) adj(a). Proof. We show that A adj(a) = det(a)i n. Let B = A adj(a) = [b ij ]. The (i, j)-entry b ij of the product matrix A adj(a) is coming from the i-th row of A and the j-th column of adj(a). Note that the j-th column of adj(a) is the cofactor of the entry a jk of the matrix A, 1 k n. b ij = n a ik C jk = a i1 C j1 + a i2 C j2 + + a in C jn. k=1

21 Fixing i. We discuss two cases. Case 1. matrices, When j = i, by the definition of determinants of b ij = det(a). Case 2. When j i, we prove that b ij = 0. Suppose that i < j; the proof for i > j is similar. We construct a matrix that differs from the matrix A only in the j-th row: the j-th row is the same as the i-th row.... a i1 a i2 a in D =.... a i1 a i2 a in...

22 Since D contains two identical rows, det(d) = 0. We expand the determinant along the j-th row: det(d) = 0 = a i1 C j1 + a i2 C j2 + a in C jn. Since D only differs from A in the j-th row, C jk = C jk. So b ij = 0, for i j. Hence the matrix B is det(a)i n. This proves the theorem.

23 Using the adjoint to find an inverse matrix. Example. A = From the previous example, the adjoint of A is computed. By the theorem, the inverse of A is det(a) The determinant of A, det(a) = 64. So the inverse is

24 Cramer s rule. Theorem. If Ax = b is a system of n linear equations in n unknowns such that det(a) 0, then the system has a unique solution. This solution is x 1 = det(a 1) det(a), x 2 = det(a 2) det(a),, x n = det(a n) det(a) when A j is the matrix obtained by replacing the entries in the j-th column of A by the entires in the matrix b 1 b 2 b =.. b n

25 If det(a) 0, then A is invertible. We multiply Ax = b by A 1 to obtain C 11 C 21 C n1 b 1 C 12 C 22 C n2 b 2 x = A 1 b = 1 det(a)... C 1n C 2n C nn. b n. We multiply the matrices on the right hand side out to obtain the i-th entry. b 1 C 1i + b 2 C 2i + + b n C ni.

26 This is precisely the determinant of the following matrix expanding along the i-th column: a 11 b 1 a 1n a 21 b 2 a 2n..... a n1 b n a nn where the i-th column of A is replaced by the column vector. So x i = det(a i) det(a).

27 Using Cramer s rule to solve a linear system. Use the Cramer s rule to solve x 1 + 2x 3 = 6, 3x 1 + 4x 2 + 6x 3 = 30, x 1 2x 2 + 3x 3 = 8.

28 and A = A 2 = , A 1 =, A 3 = The complexity here is to compute the determinants here.

29 Therefore and x 1 = det(a 1) det(a) = 10 11, x 2 = det(a 2) det(a) = x 3 = det(a 3) det(a) = We usually use the elementary row operations to compute the determinants of matrices.

30 Homework and Reading. Homework. Ex. #4, #6,#8, #10, #16, #18, #20,#24, #30, and the True-False exercise on page 116. Reading. Section 3.1.

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