# Inverses. Stephen Boyd. EE103 Stanford University. October 27, 2015

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1 Inverses Stephen Boyd EE103 Stanford University October 27, 2015

2 Outline Left and right inverses Inverse Solving linear equations Examples Pseudo-inverse Left and right inverses 2

3 Left inverses a number x that satisfies xa = 1 is called the inverse of a inverse (i.e., 1/a) exists if and only if a 0, and is unique a matrix X that satisfies XA = I is called a left inverse of A (and we say that A is left-invertible) example: the matrix has two different left inverses: B = 1 [ A = ], C = 1 [ ] Left and right inverses 3

4 Left inverse and column independence if A has a left inverse C then the columns of A are independent to see this: if Ax = 0 and CA = I then 0 = C0 = C(Ax) = (CA)x = Ix = x we ll see later the converse is also true, so a matrix is left-invertible if and only if its columns are independent so left-invertible matrices are tall or square Left and right inverses 4

5 Solving linear equations with a left inverse suppose Ax = b, and A has a left inverse C then Cb = C(Ax) = (CA)x = Ix = x so multiplying the right-hand side by a left inverse yields the solution Left and right inverses 5

6 Right inverses a matrix X that satisfies AX = I is a right inverse of A (and we say that A is right-invertible) A is right-invertible if and only if A T is left-invertible: so we conclude AX = I (AX) T = I X T A T = I A is right-invertible if and only if its rows are linearly independent right-invertible matrices are wide or square Left and right inverses 6

7 Solving linear equations with a right inverse suppose A has a right inverse B consider the (square or underdetermined) equations Ax = b x = Bb is a solution: Ax = A(Bb) = (AB)b = Ib = b so Ax = b has a solution for any b Left and right inverses 7

8 Outline Left and right inverses Inverse Solving linear equations Examples Pseudo-inverse Inverse 8

9 Inverse if A has a left and a right inverse, they are unique and equal (and we say that A is invertible) so A must be square to see this: if AX = I, Y A = I X = (Y A)X = Y (AX) = Y we denote them by A 1 : A 1 A = AA 1 = I inverse of inverse: (A 1 ) 1 = A Inverse 9

10 Solving square systems of linear equations suppose A is invertible for any b, Ax = b has the unique solution x = A 1 b matrix generalization of simple scalar equation ax = b having solution x = (1/a)b (for a 0) Inverse 10

11 Invertible matrices the following are equivalent for a square matrix A: A is invertible columns of A are linearly independent rows of A are linearly independent A has a left inverse A has a right inverse if any of these hold, all others do Inverse 11

12 Examples I 1 = I if Q is orthogonal, i.e., Q T Q = I, then Q 1 = Q T 2 2 matrix A is invertible if and only A 11 A 22 A 12 A 21 [ ] A 1 1 A22 A = 12 A 11 A 22 A 12 A 21 A 21 A 11 you need to know this formula there are similar but much more complicated formulas for larger matrices (and no, you do not need to know them) Inverse 12

13 Properties (AB) 1 = B 1 A 1 (provided inverses exist) (A T ) 1 = (A 1 ) T (sometimes denoted A T ) negative matrix powers: (A 1 ) k is denoted A k with A 0 = I, identity A k A l = A k+l holds for any integers k, l Inverse 13

14 Triangular matrices lower triangular L with nonzero diagonal entries is invertible so see this, write Lx = 0 as L 11 x 1 = 0 L 21 x 1 + L 22 x 2 = 0. L n1 x 1 + L n2 x L n,n 1 x n 1 + L nn x n = 0 from first equation, x 1 = 0 (since L 11 0) second equation reduces to L 22x 2 = 0, so x 2 = 0 (since L 22 0) and so on upper triangular R with nonzero diagonal entries is invertible Inverse 14

15 Outline Left and right inverses Inverse Solving linear equations Examples Pseudo-inverse Solving linear equations 15

16 Back substitution suppose R is upper triangular with nonzero diagonal entries write out Rx = b as R 11 x 1 + R 12 x R 1,n 1 x n 1 + R 1n x n = b 1. R n 1,n 1 x n 1 + R n 1,n x n = b n 1 R nn x n = b n from last equation we get x n = b n /R nn from 2nd to last equation we get x n 1 = (b n 1 R n 1,n x n )/R n 1,n 1 continue to get x n 2, x n 3,..., x 1 Solving linear equations 16

17 Back substitution called back substitution since we find the variables in reverse order, substituting the already known values of x i computes x = R 1 b complexity: first step requires 1 flop (division) 2nd step needs 3 flops ith step needs 2i 1 flops total is (2n 1) = n 2 flops Solving linear equations 17

18 Solving linear equations via QR factorization assuming A is invertible, let s solve Ax = b, i.e., compute x = A 1 b columns of A are linearly independent, so its QR factorization exists: A = QR Q is orthogonal, so Q 1 = Q T R is upper triangular with positive diagonal entries, hence invertible so A 1 = (QR) 1 = R 1 Q T compute x = R 1 (Q T b) by back substitution Solving linear equations 18

19 Solving linear equations via QR factorization given an n n invertible matrix A and an n-vector b 1. QR factorization. Compute the QR factorization A = QR. 2. Compute Q T b. 3. Back substitution. Solve the triangular equation Rx = Q T b using back substitution. complexity 2n 3 (step 1), 2n 2 (step 2), n 2 (step 3) total is 2n 3 + 3n 2 2n 3 Solving linear equations 19

20 Multiple right-hand sides let s solve Ax i = b i, i = 1,..., k, with A invertible carry out QR factorization once (2n 3 flops) for i = 1,..., k, solve Rx i = Q T b i via back substitution (3kn 2 flops) total is 2n 3 + 2kn 2 flops if k is small compared to n, same cost as solving one set of equations Solving linear equations 20

21 Outline Left and right inverses Inverse Solving linear equations Examples Pseudo-inverse Examples 21

22 Polynomial interpolation let s find coefficients of a cubic polynomial that satisfies p(x) = c 1 + c 2 x + c 3 x 2 + c 4 x 3 p( 1.1) = b 1, p( 0.4) = b 2, p(0.1) = b 3, p(0.8) = b 4 write as Ac = b, with A = ( 1.1) 2 ( 1.1) ( 0.4) 2 ( 0.4) (0.1) 2 (0.1) (0.8) 2 (0.8) 3 Examples 22

23 Polynomial interpolation (unique) coefficients given by c = A 1 b, with A 1 = so, e.g., c 1 is not very sensitive to b 1 or b 4 Examples 23

24 Outline Left and right inverses Inverse Solving linear equations Examples Pseudo-inverse Pseudo-inverse 24

25 Invertibility of Gram matrix A has independent columns A T A is invertible to see this, we ll show that Ax = 0 A T Ax = 0 : if Ax = 0 then (A T A)x = A T (Ax) = A T 0 = 0 : if (A T A)x = 0 then 0 = x T (A T A)x = (Ax) T (Ax) = Ax 2 = 0 so Ax = 0 Pseudo-inverse 25

26 Pseudo-inverse of tall matrix the pseudo-inverse of A with independent columns is A = (A T A) 1 A T it is a left inverse of A: A A = (A T A) 1 A T A = (A T A) 1 (A T A) = I (we ll soon see that it s a very important left inverse of A) reduces to A 1 when A is square: A = (A T A) 1 A T = A 1 A T A T = A 1 I = A 1 Pseudo-inverse 26

27 Pseudo-inverse of wide matrix if A is wide, with independent rows, AA T is invertible pseudo-inverse is defined as A = A T (AA T ) 1 A is a right inverse of A: AA = AA T (AA T ) 1 = I (we ll see later it is an important right inverse) reduces to A 1 when A is square: A T (AA T ) 1 = A T A T A 1 = A 1 Pseudo-inverse 27

28 Pseudo-inverse via QR factorization suppose A has independent columns, A = QR then A T A = (QR) T (QR) = R T Q T QR = R T R so A = (A T A) 1 A T = (R T R) 1 (QR) T = R 1 R T R T Q T = R 1 Q T can compute A using back substitution on columns of Q T for A with independent rows, A = QR T Pseudo-inverse 28

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