# 1.4 More Matrix Operations and Properties

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2 .. MORE MATRIX OPERATIONS AND PROPERTIES 9. Let A [a ij ], B [b ij ] (A + B) ij denote the the ij entry of A + B. We need to prove that (A + B) ij (B + A) ij. By de nition, (A + B) ij a ij + b ij b ij + a ij (addition of real numbers is commutative). See exercises.. See exercises.. See exercises. (B + A) ij Remark 70 Properties - are the properties of a group. Remark 7 You should notice the similarity between the properties of matrices real numbers. Properties of Scalar Multiplication Proposition 7 We assume that the sizes of the matrices involved are such that the operations listed are possible. The set of m n matrices with real coe cients together with scalar multiplication satis es the following properties:. A A A for every matrix A in the set.. (c c ) A c (c A) for every scalar c ; c every matrix A in the set.. c (A + B) ca + cb for every scalar c every matrix A B in the set.. (c + c ) A c A + c A for every scalar c ; c every matrix A in the set. Remark 7 Propositions 69 7 imply that the set of m n matrices with real coe cients together with addition scalar multiplication is a vector space. We will study vector spaces in greater details in the chapters to come. Properties of Matrix Multiplication Several important properties real numbers have with multiplication are not shared by matrices. We begin with the identity element for matrix multiplication, called the identity matrix. It is a diagonal matrix with 0 s on its diagonal. It plays a role similar to for multiplication of real numbers. Of course, there is an n n identity matrix for each value of n.

3 0 CHAPTER. SYSTEMS OF LINEAR EQUATIONS AND MATRICES Proposition 7 Suppose that A is m n recall that I n denotes the n n identity matrix (diagonal matrix with 0 s on the diagonal). Then, AI n A I m A A You will note that a di erent identity matrix was used (why?). Proof. Two matrices are equal when their corresponding entries are equal. We begin by introducing some notation. Let A [a ij ], I n [b ij ]. Let [c ij ] (AI n ) ij, the ij entry of AI n. We need to show that c ij a ij. We know by de nition that b ij 0 if i 6 j b ii. From the de nition of the product of two matrices, we have c ij mx a ik b kj k a ij (why?) The second equality is proved the same way. Proposition 75 Let A, B C be matrices with sizes such that the operations below are possible. Then, the following properties hold:. Matrix multiplication is associative, that is A (BC) (AB) C.. With 0 denoting the zero matrix, 0A A0 0.. Left distributive law: A (B + C) AB + AC.. Right distributive law: (B + C) A BA + CA. There are important properties which hold for real numbers but not for matrices. We list the most important ones.. Matrix multiplication is not commutative, though it is possible to nd some matrices for which their product will be. This is easy to see. For example, if A is m n B is n m, then AB is m m BA is n n. AB BAdo not even have the same size.. You may then ask, how about if A B are such that AB BA have the same size, for example both A B could be n n. In most cases, AB BA will still not be equal. Here is an example. 5 So, the two are not equal

4 .. MORE MATRIX OPERATIONS AND PROPERTIES. The cancellation law does not hold. Recall for real numbers if ab ac c 6 0 then a b. This is used a lot when solving equations. This does not hold for matrices. Consider 0 A 0 B The reader can verify that AB C AC So that AB AC yet A 6 B.. Another important property of real numbers is the principle of zero products which states that the product of two real numbers is zero if only if at least one of the factors is zero. It is possible for two non zero matrices to have a product equal to zero. Here is an example: We nish with an important theorem concerning the reduced row-echelon form of a matrix. Theorem 76 The reduced row-echelon form of an n n matrix either has at least a row of zeroes or is the identity matrix. Proof. This is an either or result. A common technique is to assume one of the conclusion is not true show the other has to happen. If we assume the reduced row-echelon form has no rows consisting entirely of zeroes, then every row has a leading one. Since the matrix is n n, there will be n leading ones, staggered further to the right as we move down the matrix. Since no two rows can be the same, these leading ones have to be along the diagonal.

5 CHAPTER. SYSTEMS OF LINEAR EQUATIONS AND MATRICES.. Multiplicative Inverse of a Matrix Keeping with the parallel between real numbers matrices, we know that every real number not equal to 0 has a multiplicative inverse. For matrices, it is not as simple. First, only square matrices have an inverse. Second, not every square matrix has an inverse. We begin with the de nition of the inverse of a matrix. De nition 77 Let A be an n n matrix. If there exists a matrix B, also n n such that AB BA I n then B is called the multiplicative inverse of A. The multiplicative inverse of a matrix A is usually denoted A. Remark 78 Note that the above de nition also says that A is the inverse of B. Its is important to note that we only talk about inverses for square matrices. It is also important to underst that not every square matrix has an inverse. Proposition 79 If a matrix A has an inverse, then it is unique. Proof. See exercises at the end of the section. Example 80 The inverse of we compute is , to check this, 0 Example 8 The matrix A does not have an inverse. We see 0 this by showing that no matrix canbe the inverse of A. If it did, it would a b also be a matrix. Let B. Then, we should have AB c d 0 BA. However, no matter what the entries of B are, we have 0 a b 0 a + b 0 BA. Since the second column consists c d 0 c + d 0 of zeroes, BA can never be the identity matrix. 5 5

6 .. MORE MATRIX OPERATIONS AND PROPERTIES Finding the inverse of a matrix is a long tedious process. In the next sections, we will develop a technique to do so. For matrices, there is an easy answer. We give it as a theorem. a b Theorem 8 Let A. Then A is invertible if ad bc 6 0. In this c d case, A d b ad bc c a Proof. We leave it to the reader to verify that AA A A I. Example 8 The inverse of is. We can check it by computing the product of the two We nish this subsection with a theorem regarding the inverse of a product of matrices. Theorem 8 If A B are two invertible matrices having the same size, then (AB) B A Proof. We need to show that (AB) B A B A (AB) I. Similarly (AB) B A A BB A (associativity) AIA AA I B A (AB) B A A B B IB B B I In fact, this result can be extended to a product of any number of matrices. The inverse of a product of matrices is the product of their inverses, in the reverse order

7 CHAPTER. SYSTEMS OF LINEAR EQUATIONS AND MATRICES.. Power of a Matrix Like with real numbers, we can de ne the power of a square matrix. De nition 85 If A is a square matrix, n is a positive integer, then we de ne the following:. A 0 I. A n AA A (n factors).. If A is invertible, then A n A n Example 86 Let A. Then A Example 87 Let B 0 A. Then B 0 0 B 0 0 Can you guess, then prove what B n is? The laws of exponents which hold for real numbers also hold for matrices. We give them as a theorem.

8 .. MORE MATRIX OPERATIONS AND PROPERTIES 5 Theorem 88 If A is a square matrix r s are two positive integers then A r A s A r+s Proof. See problems. Theorem 89 If A is an invertible square matrix then:. A is invertible A A.. A n is invertible (A n ) A n for n 0; ; ;. For any nonzero scalar k, (ka) is invertible (ka) k A. Proof. We prove each result separately.. Since A has an inverse which is A, we have: AA A A I. This also says that A is the inverse of A in other words A A. Since the inverse is unique, the result follows.. See problems.. We prove it by veri cation. (ka) k A k k AA AA I k A (ka) k ka A A A I We can use these de nitions properties to extend to matrices concepts which apply to real numbers. We give two as examples. Example 90 (Matrix Polynomial) Let p (x) a 0 +a x+a x + +a n x n. Let A be an n n matrix. We de ne p (A) a 0 I + a A + a A + + a n A n

9 6 CHAPTER. SYSTEMS OF LINEAR EQUATIONS AND MATRICES Example 9 (Exponential of a Matrix) Recall from Calculus II that the Taylor series for e x is As we did above, we de ne e x + x + x! + x! + e A I + A +! A +! A + The exponential of a matrix has applications in solving di erential equations... Properties of the Transpose Theorem 9 Let A B be two matrices with sizes such that the operations below are possible. Let k be any scalar. Then, the following is true:. A T T A. (A B) T A T B T. (ka) T ka T. (AB) T B T A T 5. If A is also invertible, then A T A T Proof. We prove some of these results, the remaining ones are left as exercises.. Recall that if A [a ij ] then A T a ij ji in other words, we switch the rows columns which amounts to switching the indices. We want to show that A T T a ij. If A [a ij ] then A T a ij ij ji (switch indices). Therefore, A T T a ij (switch indices again). ij. See problems. See problems. This one is a little bit more tricky, we show the proof. Let A [a ij ] be m r B [b ij ] be r n. To prove that (AB) T B T A T we need to prove the matrices have the same size the same corresponding entries. Same size. AB will be an m n matrix, so (AB) T will be an n m matrix. B T will be an n r matrix A T will be an r m matrix so that B T A T is an n m matrix. Thus (AB) T B T A T have the same size.

10 .. MORE MATRIX OPERATIONS AND PROPERTIES 7 Same corresponding entries. Let (AB) T denote the ij entry ij of (AB) T B T A T ij denote the ij entry of BT A T. We need to prove that (AB) T B T A T. We have ij ij (AB) T ij (AB) ji a j b i + a j b i + ::: + a jr b ri If we call A T ij a0 ij BT ij b0 ij, we also have B T A T ij b 0 ia 0 j + b 0 ia 0 j + + b 0 ira 0 rj So, we see the two are equal. b i a j + b i a j + + b ri a jr a j b i + a j b i + + a jr b ri 5. We prove it by veri cation. We need to show that A T A T A T A T I. A T A T AA T by # I T I A T A T A A T I T I Remark 9 The result of #can be extended to a product of more than two matrices. The transpose of a product of matrices is the product of their transpose in reverse order...5 Matrix Equations As mathematical objects, matrices can appear in equations the same way numbers do. Equations involving matrices are solved in a similar way. In this section, we only look at equations of the form Ax b where A is n n, x is n b is n. Solving the equation means nding x such that the equation is satis ed. When solving matrix equations, the following operations are permitted:

11 8 CHAPTER. SYSTEMS OF LINEAR EQUATIONS AND MATRICES. Add the same matrix on each side of the equation.. Multiply each side of the equation by the same non-zero scalar.. Multiply each side of the equation by the same non-zero matrix. Let us assume for now that A has an inverse. Then, to solve Ax b, we proceed as follows: Ax b A Ax A b (multiply each side by the same matrix) I n x A b (use the fact that A A I n ) x A b (property of the identity matrix) Of course, the above assumes that A has an inverse. Application: Linear System of Equation Written as a Matrix Equation One technique used to solve a linear system of n equations in n variables is to write it as a matrix equation, then solve the matrix equation. It is easy to see that the above system is equivalent to the matrix equation Ax b where A 6 a a a n a a a n x b a n a n a nn x x x n b b b n A is called the coe cient matrix, b is called the constant matrix. Therefore, the solution, when it exists, is x A b It should be noted that this is not the preferred way to solve a system. Finding the inverse of a matrix is di cult time consuming. There are better methods.

12 .. MORE MATRIX OPERATIONS AND PROPERTIES 9 Remark 9 With the notation of this section, the augmented matrix for a system can be denoted a a a n j b a a a n j b A j b j 5 a n a n a nn j b n..6 Problems on Sections. Finish proving proposition 7.. Prove proposition 79 that is prove that if a matrix has an inverse, then the inverse is unique. Hint: assume there are two inverses show they are equal.. Finish proving the properties in propositions Finish proving proposition Think of problems in your areas of interest in which solving the problem would involve solving a linear system of equations Find B n for any positive integer n for B. You must prove your claim. 7. Prove theorem Finish proving theorem Finish proving theorem On pages 8-5 of your book, do #,, 5, 6, 7, 8,,,, 5, 6, 7,, 9,.

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