SVD: Singular Value Decomposition

1. SVD: Singular Value Decomposition

2. Motivation Assume A full rank Clearly the winner

3. row space column space Ax=0 ATy=0 Ideas Behind SVD There are many choices of basis in C(AT) and C(A), but we want the orthonormal ones • Goal: for Am×n • find orthonormal bases for C(AT) and C(A) orthonormal basis in C(A) orthonormal basis in C(AT) A Rm Rn

4. SVD (2X2) I haven’t told you how to find vi’s (p.9) s: represent the length of images; hence non-negative

5. SVD 2x2 (cont) Another diagonalization using 2 sets of orthogonal bases Compare When A has complete set of e-vectors, we have AS=SL , A=SLS-1 but S in general is not orthogonal When A is symmetric, we have A=QLQT

6. ( )-1=( )T Implication: Matrix inversion Ax=b Why are orthonormal bases good?

7. More on U and V V: eigenvectors of ATA U: eigenvectors of AAT [Find vi first, then use Avi to find ui This is the key to solve SVD

8. SVD: A=USVT • The singular values are the diagonal entries of the S matrix and are arranged in descending order • The singular values are always real (non-negative) numbers • If A is real matrix, U and V are also real

9. Example (2x2, full rank) • STEPS: • Find e-vectors of ATA; normalize the basis • Compute Avi, get si • If si  0, get ui • Else find ui from N(AT)

10. SVD Theory • If sj=0, Avj=0vj is in N(A) • The corresponding uj in N(AT) • [UTA=SVT=0] • Else, vj in C(AT) • The corresponding uj in C(A) • #of nonzero sj = rank

11. Example (2x2, rank deficient) Can also be obtained from e-vectors of ATA

12. Example (cont) Bases of N(A) and N(AT) (u2 and v2 here) do not contribute the final result. The are computed to make U and V orthogonal.

13. Basis of N(A) Basis of N(AT) Extend to Amxn Dimension Check

14. Extend to Amxn (cont) Summation of r rank-one matrices! Bases of N(A) and N(AT) do not contribute They are useful only for nullspace solutions

15. N(A) C(A) C(AT)

17. Summary • SVD chooses the right basis for the 4 subspaces • AV=US • v1…vr: orthonormal basis in Rn for C(AT) • vr+1…vn: N(A) • u1…ur: in Rm C(A) • ur+1…um: N(AT) • These bases are not only , but also Avi=siui • High points of Linear Algebra • Dimension, rank, orthogonality, basis, diagonalization, …

18. SVD Applications • Using SVD in computation, rather than A, has the advantage of being more robust to numerical error • Many applications: • Inverse of matrix A • Conditions of matrix • Image compression • Solve Ax=b for all cases (unique, many, no solutions; least square solutions) • rank determination, matrix approximation, … • SVD usually found by iterative methods (see Numerical Recipe, Chap.2)

20. SVD and Ax=b (mn) • Check for existence of solution

21. Ax=b (inconsistent) No solution!

22. Ax=b (underdetermined)

23. Pseudo Inverse (Sec7.4, p.395) • The role of A: • Takes a vector vi from row space to siui in the column space • The role of A-1 (if it exists): • Does the opposite: takes a vector ui from column space to row space vi

24. Pseudo Inverse (cont) • While A-1 may not exist, a matrix that takes ui back to vi/si does exist. It is denoted as A+, the pseudo inverse • A+: dimension n by m

25. Pseudo Inverse and Ax=b • Overdetermined case: find the solution that minimize the error r=|Ax–b|, the least square solution A panacea for Ax=b

26. Ex: full rank

27. Will show this need not be computed… Ex: over-determined

28. Over-determined (cont) Same result!!

29. Ex: general case, no solution

30. Matrix Approximation making small s’s to zero and back substitute (see next page for application in image compression)

31. Image Compression • As described in text p.352 • For grey scale images: mn bytes • Only need to store r(m+n+1) Original 6464 r = 1,3,5,10,16 (no perceivable difference afterwards)