# Section 4.5. Matrix Inverses

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1 Section 4.5 Matrix Inverses

2 The Definition of Inverse Recall: The multiplicative inverse (or reciprocal) of a nonzero number a is the number b such that ab = 1. We define the inverse of a matrix in almost the same way. Definition Let A be an n n square matrix. We say A is invertible (or nonsingular) if there is a matrix B of the same size, such that identity matrix AB = I n and BA = I n In this case, B is the inverse of A, and is written A 1... Example A = ( ) B = ( ) I claim B = A 1. Check: ( ) ( ) ( ) AB = = ( ) ( ) ( ) " BA = =

3 Poll Poll Do there exist two matrices A and B such that AB is the identity, but BA is not? If so, find an example. (Both products have to make sense.) Yes, for instance: A = ( 1 0 ) B = However If A and B are square matrices, then AB = I n if and only if BA = I n. So in this case you only have to check one.

4 Solving Linear Systems via Inverses Solving Ax = b by dividing by A Theorem If A is invertible, then Ax = b has exactly one solution for every b, namely: x = A 1 b. Why? Divide by A! Ax = b A 1 (Ax) = A 1 b (A 1 A)x = A 1 b I nx = A 1 b x = A 1 b. I nx = x for every x Important If A is invertible and you know its inverse, then the easiest way to solve Ax = b is by dividing by A : x = A 1 b. This is very convenient when you have to vary b!

5 Solving Linear Systems via Inverses Example Example Solve the system 2x 1 + 3x 2 + 2x 3 = 1 x 1 + 3x 3 = 1 2x 1 + 2x 2 + 3x 3 = 1 Answer: using x x 2 = x = = = The advantage of using inverses is it doesn t matter what s on the right-hand side of the = : 2x 1 + 3x 2 + 2x 3 = b 1 1 x b 1 x 1 + 3x 3 = b 2 = x 2 = b 2 2x 1 + 2x 2 + 3x 3 = b x b 3 3 6b 1 5b 2 + 9b 3 = 3b 1 + 2b 2 4b 3. 2b 1 + 2b 2 3b 3

6 Some Facts Say A and B are invertible n n matrices. 1. A 1 is invertible and its inverse is (A 1 ) 1 = A. 2. AB is invertible and its inverse is (AB) 1 = A 1 B 1 B 1 A 1. Why? (B 1 A 1 )AB = B 1 (A 1 A)B = B 1 I nb = B 1 B = I n. 3. A T is invertible and (A T ) 1 = (A 1 ) T. Why? A T (A 1 ) T = (A 1 A) T = In T = I n. Question: If A, B, C are invertible n n matrices, what is the inverse of ABC? i. A 1 B 1 C 1 ii. B 1 A 1 C 1 iii. C 1 B 1 A 1 iv. C 1 A 1 B 1 Check: (ABC)(C 1 B 1 A 1 ) = AB(CC 1 )B 1 A 1 = A(BB 1 )A 1 = AA 1 = I n. In general, a product of invertible matrices is invertible, and the inverse is the product of the inverses, in the reverse order.

7 Computing A 1 The 2 2 case ( ) a b Let A =. The determinant of A is the number c d ( ) a b det(a) = det = ad bc. c d Facts: 1. If det(a) 0, then A is invertible and A 1 = 1 det(a) 2. If det(a) = 0, then A is not invertible. Why 1? ( ) ( ) a b d b = c d c a ( ) ad bc 0 = 0 ad bc So we get the identity by dividing by ad bc. ( d b c a ). ( ) ( ) d b a b c a c d Example ( ) 1 2 det = = ( ) = ( )

8 Computing A 1 In general Let A be an n n matrix. Here s how to compute A Row reduce the augmented matrix ( A I n ). 2. If the result has the form ( I n B ), then A is invertible and B = A Otherwise, A is not invertible. Example A = [interactive]

9 Computing A 1 Example R 3 = R 3 + 3R R 1 = R 1 2R 3 R 2 = R 2 R R 3 = R /2 1/ So = /2 1/2 Check: = " /2 1/

10 Why Does This Work? We can think of the algorithm as simultaneously solving the equations Ax 1 = e 1 : Ax 2 = e 2 : Ax 3 = e 3 : Now note A 1 e i = A 1 (Ax i ) = x i, and x i is the ith column in the augmented part. Also A 1 e i is the ith column of A 1.

11 Invertible Transformations Definition A transformation T : R n R n is invertible if there exists another transformation U : R n R n such that T U(x) = x and U T (x) = x for all x in R n. In this case we say U is the inverse of T, and we write U = T 1. In other words, T (U(x)) = x, so T undoes U, and likewise U undoes T. Fact A transformation T is invertible if and only if it is both one-to-one and onto. If T is one-to-one and onto, this means for every y in R n, there is a unique x in R n such that T (x) = y. Then T 1 (y) = x.

12 Invertible Transformations Examples Let T = counterclockwise rotation in the plane by 45. What is T 1? T T 1 T 1 is clockwise rotation by 45. [interactive: T 1 T ] [interactive: T T 1 ] Let T = shrinking by a factor of 2/3 in the plane. What is T 1? T T 1 T 1 is stretching by 3/2. [interactive: T 1 T ] [interactive: T T 1 ] Let T = projection onto the x-axis. What is T 1? It is not invertible: you can t undo it.

13 Invertible Linear Transformations If T : R n R n is an invertible linear transformation with matrix A, then what is the matrix for T 1? Let B be the matrix for T 1. We know T T 1 has matrix AB, so for all x, Hence AB = I n, so B = A 1. ABx = T T 1 (x) = x. Fact If T is an invertible linear transformation with matrix A, then T 1 is an invertible linear transformation with matrix A 1.

14 Invertible Linear Transformations Examples Let T = counterclockwise rotation in the plane by 45. Its matrix is ( ) cos(45 ) sin(45 ) A = sin(45 ) cos(45 = 1 ( ) 1 1. ) Then T 1 = counterclockwise rotation by 45. Its matrix is ( ) cos( 45 ) sin( 45 ) B = sin( 45 ) cos( 45 = 1 ( ) 1 1. ) Check: AB = 1 ( ) ( ) ( ) = Let T = shrinking by a factor of 2/3 in the plane. Its matrix is ( ) 2/3 0 A = 0 2/3 Then T 1 = stretching by 3/2. Its matrix is ( ) 3/2 0 B = 0 3/2 ( ) ( ) 2/3 0 3/2 0 Check: AB = = 0 2/3 0 3/2 ( ) " "

15 The Invertible Matrix Theorem A.K.A. The Really Big Theorem of Math 1553 The Invertible Matrix Theorem Let A be an n n matrix, and let T : R n R n be the linear transformation T (x) = Ax. The following statements are equivalent. 1. A is invertible. 2. T is invertible. 3. The reduced row echelon form of A is the identity matrix I n. 4. A has n pivots. 5. Ax = 0 has no solutions other than the trivial solution. 6. Nul(A) = {0}. 7. nullity (A) = The columns of A are linearly independent. 9. The columns of A form a basis for R n. 10. T is one-to-one. 11. Ax = b is consistent for all b in R n. 12. Ax = b has a unique solution for each b in R n. 13. The columns of A span R n. 14. Col A = R n. 15. dim Col A = n. 16. rank A = n. 17. T is onto. 18. There exists a matrix B such that AB = I n. 19. There exists a matrix B such that BA = I n. you really have to know these

16 The Invertible Matrix Theorem Summary There are two kinds of square matrices: 1. invertible (non-singular), and 2. non-invertible (singular). For invertible matrices, all statements of the Invertible Matrix Theorem are true. For non-invertible matrices, all statements of the Invertible Matrix Theorem are false. Strong recommendation: If you want to understand invertible matrices, go through all of the conditions of the IMT and try to figure out on your own (or at least with help from the book) why they re all equivalent. You know enough at this point to be able to reduce all of the statements to assertions about the pivots of a square matrix.

17 The Invertible Matrix Theorem Example Question: Is this matrix invertible? A = The second column is a multiple of the first, so the columns are linearly dependent. A does not satisfy condition (8) of the IMT, so it is not invertible.

18 The Invertible Matrix Theorem Another Example Problem: Let A be a 3 3 matrix such that 1 2 A 7 = A Show that the rank of A is at most 2. If we set 1 2 b = A 7 = A 0, 0 1 then Ax = b has multiple solutions, so it does not satisfy condition (12) of the IMT. Hence it also does not satisfy condition (16), so the rank is not 3. In any case the rank is at most 3, so it must be less than 3.

19 Summary The inverse of a square matrix A is a matrix A 1 such that AA 1 = I n (equivalently, A 1 A = I n). If A is invertible, then you can solve Ax = b by dividing by A : b = A 1 x. There is a unique solution x = A 1 b for every x. You compute A 1 (and whether A is invertible) by row reducing ( A ) I n. There s a trick for computing the inverse of a 2 2 matrix in terms of determinants. A linear transformation T is invertible if and only if its matrix A is invertible, in which case A 1 is the matrix for T 1. The Invertible Matrix theorem is a list of a zillion equivalent conditions for invertibility that you have to learn (and should understand, since it s well within what we ve covered in class so far).

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