4. Matrix inverses. left and right inverse. linear independence. nonsingular matrices. matrices with linearly independent columns

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1 L. Vandenberghe EE133A (Spring 2016) 4. Matrix inverses left and right inverse linear independence nonsingular matrices matrices with linearly independent columns matrices with linearly independent rows 4-1

2 Left and right inverse AB BA in general, so we have to distinguish two types of inverses Left inverse: X is a left inverse of A if XA = I A is left invertible if it has at least one left inverse Right inverse: X is a right inverse of A if AX = I A is right invertible if it has at least one right inverse Matrix inverses 4-2

3 Examples A = , B = [ ] A is left invertible; the following matrices are left inverses: 1 9 [ ], [ 0 1/ /2 2 ] B is right invertible; the following matrices are right inverses: , , Matrix inverses 4-3

4 Some immediate properties Dimensions a left or right inverse of an m n matrix must have size n m Left and right inverse of (conjugate) transpose X is a left inverse of A if and only if X T is a right inverse of A T A T X T = (XA) T = I X is a left inverse of A if and only if X H is a right inverse of A H A H X H = (XA) H = I Matrix inverses 4-4

5 Inverse if A has a left and a right inverse, then they are equal and unique: XA = I, AY = I = X = X(AY ) = (XA)Y = Y we call X = Y the inverse of A (notation: A 1 ) A is invertible if its inverse exists Example A = , A 1 = Matrix inverses 4-5

6 Linear equations set of m linear equations in n variables A 11 x 1 + A 12 x A 1n x n = b 1 A 21 x 1 + A 22 x A 2n x n = b 2.. A m1 x 1 + A m2 x A mn x n = b m in matrix form: Ax = b may have no solution, a unique solution, infinitely many solutions Matrix inverses 4-6

7 Linear equations and matrix inverse Left invertible matrix: if X is a left inverse of A, then Ax = b = x = XAx = Xb there is at most one solution (if there is a solution, it must be equal to Xb) Right invertible matrix: if X is a right inverse of A, then x = Xb = Ax = AXb = b there is at least one solution (namely, x = Xb) Invertible matrix: if A is invertible, then Ax = b x = A 1 b there is a unique solution Matrix inverses 4-7

8 Outline left and right inverse linear independence nonsingular matrices matrices with linearly independent columns matrices with linearly independent rows

9 Linear combination a linear combination of vectors a 1,..., a n is a sum of scalar products x 1 a 1 + x 2 a x n a n the scalars x i are the coefficients of the linear combination can be written as a matrix-vector product x 1 a 1 + x 2 a x n a n = [ ] a 1 a 2 a n x 1 x 2.. x n the trivial linear combination has coefficients x 1 = = x n = 0 (same definition holds for real and complex vectors/scalars) Matrix inverses 4-8

10 Linear dependence a set of vectors a 1, a 2,..., a n is linearly dependent if x 1 a 1 + x 2 a x n a n = 0 for some scalars x 1,..., x n, not all zero vector 0 can be written as a nontrivial linear combination of a 1,..., a n equivalently, at least one vector a i is a linear combination of the rest: if x i 0 a i = x 1 x i a 1 x i 1 x i a i 1 x i+1 x i a i+1 x n x i a n Matrix inverses 4-9

11 Example the vectors a 1 = , a 2 = , a 3 = are linearly dependent 0 can be expressed as a nontrivial linear combination of a 1, a 2, a 3 : 0 = a 1 + 2a 2 3a 3 a 1 can be expressed as a linear combination of a 2, a 3 : (and similarly a 2 and a 3 ) a 1 = 2a 1 + 3a 3 Matrix inverses 4-10

12 Linear independence a 1,..., a n are linearly independent if they are not linearly dependent the zero vector cannot be written as a nontrivial linear combination: x 1 a 1 + x 2 a x n a n = 0 = x 1 = x 2 = = x n = 0 none of the vectors a i is a linear combination of the other vectors Matrix with linearly independent columns A = [ a 1 a 2 a n ] has linearly independent columns if Ax = 0 only for x = 0 Matrix inverses 4-11

13 Example the vectors a 1 = 1 2 0, a 2 = 1 0 1, a 3 = are linearly independent: only if x 1 = x 2 = x 3 = 0 x 1 a 1 + x 2 a 2 + x 3 a 3 = x 1 x 2 2x 1 + x 3 x 2 + x 3 = 0 Matrix inverses 4-12

14 Dimension inequality if n vectors a 1, a 2,..., a n of length m are linearly independent, then n m (proof is in textbook) if an m n matrix has linearly independent columns then m n if an m n matrix has linearly independent rows then m n Matrix inverses 4-13

15 Outline left and right inverse linear independence nonsingular matrices matrices with linearly independent columns matrices with linearly independent rows

16 Nonsingular matrix for a square matrix A the following four properties are equivalent 1. A is left invertible 2. the columns of A are linearly independent 3. A is right invertible 4. the rows of A are linearly independent a square matrix with these properties is called nonsingular Nonsingular = invertible if properties 1 and 3 hold, then A is invertible (page 4-5) if A is invertible, properties 1 and 3 hold (by definition of invertibility) Matrix inverses 4-14

17 Proof left invertible (b ) (a) linearly independent columns (b) linearly independent rows (a ) right invertible we show that (a) holds in general we show that (b) holds for square matrices (a ) and (b ) follow from (a) and (b) applied to A T Matrix inverses 4-15

18 Part a: suppose A is left invertible if B is a left inverse of A (satisfies BA = I), then Ax = 0 = BAx = 0 = x = 0 this means that the columns of A are linearly independent: if A = [ a 1 a 2 a n ] then x 1 a 1 + x 2 a x n a n = 0 holds only for the trivial linear combination x 1 = x 2 = = x n = 0 Matrix inverses 4-16

19 Part b: suppose A is square with linearly independent columns a 1,..., a n for every n-vector b the vectors a 1,..., a n, b are linearly dependent (from inequality on page 4-13) hence for every b there exists a nontrivial linear combination x 1 a 1 + x 2 a x n a n + x n+1 b = 0 we must have x n+1 0 because a 1,..., a n are linearly independent hence every b can be written as a linear combination of a 1,..., a n in particular, there exist n-vectors c 1,..., c n such that Ac 1 = e 1, Ac 2 = e 2,..., Ac n = e n, the matrix C = [ ] c 1 c 2 c n is a right inverse of A: A [ ] [ ] c 1 c 2 c n = e1 e 2 e n = I Matrix inverses 4-17

20 Examples A = , B = A is nonsingular because its columns are linearly independent: x 1 x 2 + x 3 = 0, x 1 + x 2 + x 3 = 0, x 1 + x 2 x 3 = 0 is only possible if x 1 = x 2 = x 3 = 0 B is singular because its columns are linearly dependent: Bx = 0 for x = (1, 1, 1, 1) Matrix inverses 4-18

21 Example: Vandermonde matrix A = 1 t 1 t 2 1 t n t 2 t 2 2 t n t n t 2 n t n 1 n with t i t j for i j we show that A is nonsingular by showing that Ax = 0 only if x = 0 Ax = 0 means p(t 1 ) = p(t 2 ) = = p(t n ) = 0 where p(t) = x 1 + x 2 t + x 3 t x n t n 1 p(t) is a polynomial of degree n 1 or less if x 0, then p(t) can not have more than n 1 distinct real roots therefore p(t 1 ) = = p(t n ) = 0 is only possible if x = 0 Matrix inverses 4-19

22 Inverse of transpose and product Transpose and conjugate transpose if A is nonsingular, then A T and A H are nonsingular and (A T ) 1 = (A 1 ) T, (A H ) 1 = (A 1 ) H, we write these as A T and A H Product if A and B are nonsingular and of equal size, then AB is nonsingular with (AB) 1 = B 1 A 1 Matrix inverses 4-20

23 Outline left and right inverse linear independence nonsingular matrices matrices with linearly independent columns matrices with linearly independent rows

24 Gram matrix A = [ a 1 a 2 a n ] the Gram matrix associated with A is the matrix of column inner products for real matrices: A T A = a T 1 a 1 a T 1 a 2 a T 1 a n a T 2 a 1 a T 2 a 2 a T 2 a n... a T na 1 a T na 2 a T na n for complex matrices A H A = a H 1 a 1 a H 1 a 2 a H 1 a n a H 2 a 1 a H 2 a 2 a H 2 a n... a H n a 1 a H n a 2 a H n a n Matrix inverses 4-21

25 Nonsingular Gram matrix Gram matrix is nonsingular if only if A has linearly independent columns suppose A R m n has linearly independent columns: A T Ax = 0 = x T A T Ax = (Ax) T (Ax) = Ax 2 = 0 = Ax = 0 = x = 0 therefore A T A is nonsingular suppose the columns of A R m n are linearly dependent x 0, Ax = 0 = x 0, A T Ax = 0 therefore A T A is singular (for A C m n, replace A T with A H and x T with x H ) Matrix inverses 4-22

26 Pseudo-inverse suppose A R m n has linearly independent columns this implies that A is tall or square (m n); see page 4-13 the pseudo-inverse of A is defined as A = (A T A) 1 A T this matrix exists, because the Gram matrix A T A is nonsingular A is a left inverse of A: A A = (A T A) 1 (A T A) = I (for complex A with linearly independent columns, A = (A H A) 1 A H ) Matrix inverses 4-23

27 Summary the following three properties are equivalent 1. A is left invertible 2. the columns of A are linearly independent 3. A T A is nonsingular 1 2 was already proved on page : we have seen that the pseudo-inverse is a left inverse 2 3: proved on page 4-22 a matrix with these properties must be tall or square Matrix inverses 4-24

28 Outline left and right inverse linear independence nonsingular matrices matrices with linearly independent columns matrices with linearly independent rows

29 Pseudo-inverse suppose A R m n has linearly independent rows this implies that A is wide or square (m n); see page 4-13 the pseudo-inverse of A is defined as A = A T (AA T ) 1 A T has linearly independent columns hence its Gram matrix AA T is nonsingular, so A exists A is a right inverse of A: AA = (AA T )(AA T ) 1 = I (for complex A with linearly independent rows, A = A H (AA H ) 1 ) Matrix inverses 4-25

30 Summary the following three properties are equivalent 1. A is right invertible 2. the rows of A are linearly independent 3. AA T is nonsingular 1 2 and 2 3: by transposing result on page : we have seen that the pseudo-inverse is a right inverse a matrix with these properties must be wide or square Matrix inverses 4-26

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