Orthogonality and Least Squares

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1 6 Orthogonality and Least Squares 6.5 LEAS-SQUARES S

2 LEAS-SQUARES S Definition: If A is and is in, a leastsquares solution of is an in such n that n for all x in. m n A x = x Ax Ax m he most important aspect of the least-squares prolem is that no matter what x we select, the vector Ax will necessarily e in the column space, Col A. So we seek an x that makes Ax the closest point in Col A to. See the figure on the next slide. Slide 6.5-2

3 LEAS-SQUARES S Solution of the General Least-Squares Prolem Given A and, apply the Best Approximation heorem to the suspace Col A. Let = proj Col A Slide 6.5-3

4 SOLUION OF HE GENREAL LEAS-SQUARES Because is in the column space A, the equation n is consistent, and there is an in such that A x = x A x = ----(1) Since is the closest point in Col A to, a vector is a least-squares solution of A x = if and only if x satisfies (1). x n Such an in is a list of weights that will uild out of the columns of A. See the figure on the next slide. x Slide 6.5-4

5 SOLUION OF HE GENREAL LEAS-SQUARES x A x = Suppose satisfies. By the Orthogonal Decomposition heorem, the projection has the property that is orthogonal to Col A, so is orthogonal to each column of A. Ax If a j is any column of A, then, j and. a ( Ax) j a ( Ax) = 0 Slide 6.5-5

6 SOLUION OF HE GENREAL LEAS-SQUARES Since each is a row of A, hus a j A A ( Ax) = 0 A Ax = 0 A Ax ----(2) hese calculations show that each least-squares solution of A x = satisfies the equation A Ax = A ----(3) he matrix equation (3) represents a system of equations called the normal equations for A x =. A solution of (3) is often denoted y. = A x Slide 6.5-6

7 SOLUION OF HE GENREAL LEAS-SQUARES A x = heorem 13: he set of least-squares solutions of coincides with the nonempty set of solutions of the normal equation A Ax = A. Proof: he set of least-squares solutions is nonempty and each least-squares solution x satisfies the normal equations. Conversely, suppose x satisfies A Ax = A. hen x satisfies (2), which shows that Ax is orthogonal to the rows of A and hence is orthogonal to the columns of A. Since the columns of A span Col A, the vector Ax is orthogonal to all of Col A. Slide 6.5-7

8 SOLUION OF HE GENREAL LEAS-SQUARES Hence the equation = Ax + ( Ax) is a decomposition of into the sum of a vector in Col A and a vector orthogonal to Col A. By the uniqueness of the orthogonal decomposition, must e the orthogonal projection of onto Col A. Ax A x = x hat is, and is a least-squares solution. Slide 6.5-8

9 SOLUION OF HE GENREAL LEAS-SQUARES Example 1: Find a least-squares solution of the inconsistent system for Solution: o use normal equations (3), compute: A A A x = A = 0 2, = = = Slide 6.5-9

10 SOLUION OF HE GENREAL LEAS-SQUARES A = = hen the equation A Ax = A ecomes 17 1 x = x 2 11 Slide

11 SOLUION OF HE GENREAL LEAS-SQUARES Row operations can e used to solve the system on the previous slide, ut since A A is invertile and 2 2, it is proaly faster to compute and then solve ( A A) = A Ax = A 1 x = ( A A) A as = 84 = = Slide

12 SOLUION OF HE GENREAL LEAS-SQUARES m n heorem 14: Let A e an matrix. he following statements are logically equivalent: a. he equation A x = has a unique least-squares m solution for each in.. he columns of A are linearly independent. c. he matrix A A is invertile. When these statements are true, the least-squares x solution is given y 1 x ( = A A) A ----(4) When a least-squares solution x is used to produce A as an approximation to, the distance from to Ax is called the least-squares error of this approximation. x Slide

13 ALERNAIVE CALCULAIONS OF LEAS- SQUARES SOLUIONS Example 2: Find a least-squares solution of A =, = A x = Solution: Because the columns a 1 and a 2 of A are orthogonal, the orthogonal projection of onto Col A is given y a a a a 8 a 45 = + = + a a a a 4 90 a for ----(5) Slide

14 ALERNAIVE CALCULAIONS OF LEAS- SQUARES SOLUIONS Now that is known, we can solve. But this is trivial, since we already know weights to place on the columns of A to produce. It is clear from (5) that 8 / 4 2 x = 45 / 90 = 1/ = + = 2 1/ 2 5 / / 2 11/ 2 A x = Slide

15 ALERNAIVE CALCULAIONS OF LEAS- SQUARES SOLUIONS heorem 15: Given an matrix A with linearly independent columns, let A = QR e a QR factorization m of A. hen, for each in, the equation A x = has a unique least-squares solution, given y Proof: Let. x = x = 1 R Q m n 1 R Q ----(6) hen A QR QRR Q QQ 1 x = x = = Slide

16 ALERNAIVE CALCULAIONS OF LEAS- SQUARES SOLUIONS he columns of Q form an orthonormal asis for Col A. Hence, y heorem 10, QQ is the orthogonal projection of onto Col A. A x = A x = hen, which shows that is a least-squares solution of. x he uniqueness of follows from heorem 14. x Slide

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