Section 6.5. Least Squares Problems

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1 Section 6.5 Least Squares Problems

2 Motivation We now are in a position to solve the motivating problem of this third part of the course: Problem Suppose that Ax = b does not have a solution. What is the best possible approximate solution? To say Ax = b does not have a solution means that b is not in Col A. The closest possible b for which Ax = b does have a solution is b = proj Col A (b). Then A x = b is a consistent equation. A solution x to A x = b is a least squares solution.

3 Least Squares Solutions Let A be an m n matrix. Definition A least squares solution of Ax = b is a vector x in R n such that for all x in R n. b A x b Ax Note that b A x is in (Col A). b b A x Ax Ax Ax Col A [interactive] A x = b = proj Col A (b) In other words, a least squares solution x solves Ax = b as closely as possible. Equivalently, a least squares solution to Ax = b is a vector x in R n such that A x = b = proj Col A (b). This is because b is the closest vector to b such that A x = b is consistent.

4 Least Squares Solutions Computation Theorem The least squares solutions to Ax = b are the solutions to (A T A) x = A T b. This is just another Ax = b problem, but with a square matrix A T A! Note we compute x directly, without computing b first. Why is this true? We want to find x such that A x = proj Col A (b). This means b A x is in (Col A). Recall that (Col A) = Nul(A T ). So b A x is in (Col A) if and only if A T (b A x) = 0. In other words, A T A x = A T b. Alternative when A has orthogonal columns v 1, v 2,..., v n: n b v i b = proj Col A (b) = v i v i v i i=1 ( b v1 The right hand side equals A x, where x =, v 1 v 1 b v 2 v 2 v 2,, ) b v n. v n v n

5 Least Squares Solutions Example Find the least squares solutions to Ax = b where: A = 1 1 b = We have and A T A = A T b = ( Row reduce: ( ( ) = 1 2 ) 6 0 = 0 ( ) ( ) 6. 0 ) ( ( ) 5 So the only least squares solution is x =. 3 ).

6 Least Squares Solutions Example, continued How close did we get? 1 0 ( ) 5 b = A x = = The distance from b is 6 5 b A x = = = ( 2) = 6. z y x [interactive] v 2 5v 1 6 3v 2 v 1 Col A b ( ) 5 b = A 3 Let 1 v 1 = 1 and 0 v 2 = be the columns of A, and let B = {v 1, v 2}. Note x = ( 5 3) is just the B-coordinates of b, in Col A = Span{v1, v 2}.

7 Least Squares Solutions Second example Find the least squares solutions to Ax = b where: A = 1 1 b = We have and A T A = ( ) = A T b = ( ) = ( 5 ) ( ) 2. 2 Row reduce: ( ) ( ) / /3 ( ) 1/3 So the only least squares solution is x =. [interactive] 1/3

8 Least Squares Solutions Uniqueness When does Ax = b have a unique least squares solution x? Theorem Let A be an m n matrix. The following are equivalent: 1. Ax = b has a unique least squares solution for all b in R n. 2. The columns of A are linearly independent. 3. A T A is invertible. In this case, the least squares solution is (A T A) 1 (A T b). Why? If the columns of A are linearly dependent, then A x = b has many solutions: v 2 v 1 b [interactive] v 3 Col A b = A x Note: A T A is always a square matrix, but it need not be invertible.

9 Application Data modeling: best fit line Find the best fit line through (0, 6), (1, 0), and (2, 0). The general equation of a line is So we want to solve: y = C + Dx. 6 = C + D 0 0 = C + D 1 0 = C + D 2. In matrix form: 1 0 ( ) C = 0. D [interactive] (0, 6) 1 y = 3x (2, 0) (1, 0) 1 We ( already saw: the least squares solution is 5 ) 3. So the best fit line is y = 3x + 5. ( ) 5 A 6 0 =

10 Poll Poll What does the best fit line minimize? A. The sum of the squares of the distances from the data points to the line. B. The sum of the squares of the vertical distances from the data points to the line. C. The sum of the squares of the horizontal distances from the data points to the line. D. The maximal distance from the data points to the line. Answer: B. See the picture on the previous slide.

11 Application Best fit ellipse Find the best fit ellipse for the points (0, 2), (2, 1), (1, 1), ( 1, 2), ( 3, 1), ( 1, 1). The general equation for an ellipse is So we want to solve: x 2 + Ay 2 + Bxy + Cx + Dy + E = 0 (0) 2 + A(2) 2 + B(0)(2) + C(0) + D(2) + E = 0 (2) 2 + A(1) 2 + B(2)(1) + C(2) + D(1) + E = 0 (1) 2 + A( 1) 2 + B(1)( 1) + C(1) + D( 1) + E = 0 ( 1) 2 + A( 2) 2 + B( 1)( 2) + C( 1) + D( 2) + E = 0 ( 3) 2 + A(1) 2 + B( 3)(1) + C( 3) + D(1) + E = 0 ( 1) 2 + A( 1) 2 + B( 1)( 1) + C( 1) + D( 1) + E = 0 In matrix form: A B C D = E

12 Application Best fit ellipse, continued A = A T A = Row reduce: Best fit ellipse: or b = A T b = / / / / /133 x y xy x y = 0 266x y 2 178xy + 402x 123y 1374 = 0.

13 Application Best fit ellipse, picture [interactive] (0, 2) ( 3, 1) ( 1, 1) (2, 1) (1, 1) ( 1, 2) 266x y 2 178xy + 402x 123y 1374 = 0 Remark: Gauss invented the method of least squares to do exactly this: he predicted the (elliptical) orbit of the asteroid Ceres as it passed behind the sun in 1801.

14 Application Best fit parabola What least squares problem Ax = b finds the best parabola through the points ( 1, 0.5), (1, 1), (2, 0.5), (3, 2)? The general equation for a parabola is So we want to solve: In matrix form: y = Ax 2 + Bx + C. 0.5 = A( 1) 2 + B( 1) + C 1 = A(1) 2 + B(1) + C 0.5 = A(2) 2 + B(2) + C 2 = A(3) 2 + B(3) + C A B = C Answer: 88y = 53x x 82

15 Application Best fit parabola, picture 88y = 53x x 82 (3, 2) ( 1, 0.5) (1, 1) (2, 0.5) [interactive]

16 Application Best fit linear function What least squares problem Ax = b finds the best linear function f (x, y) fitting the following data? The general equation for a linear function in two variables is f (x, y) = Ax + By + C. x y f (x, y) So we want to solve In matrix form: A(1) + B(0) + C = 0 A(0) + B(1) + C = 1 A( 1) + B(0) + C = 3 A(0) + B( 1) + C = A B = 1 3. C Answer: f (x, y) = 3 2 x 3 2 y + 2

17 Application Best fit linear function, picture [interactive] (0, 1, 4) f (x, y) f ( 1, 0) f (0, 1) ( 1, 0, 3) Graph of y x (0, 1, 1) f (1, 0) f (0, 1) (1, 0, 0) f (x, y) = 3 2 x 3 2 y + 2

18 Summary A least squares solution of Ax = b is a vector x such that b = A x is as close to b as possible. This means that b = proj Col A (b). One way to compute a least squares solution is by solving the system of equations (A T A) x = A T b. Note that A T A is a (symmetric) square matrix. Least-squares solutions are unique when the columns of A are linearly independent. You can use least-squares to find best-fit lines, parabolas, ellipses, planes, etc.

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