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  1. Optimal Column-Based Low-Rank Matrix Reconstruction SODA’12 Ali Kemal Sinop Joint work with Prof. Venkatesan Guruswami

  2. Outline • Introduction • Notation • Problem Definition • Motivations • Results • Upper Bound • Randomized Algorithm • Summary SODA 2012: Ali Kemal Sinop

  3. Notation: Vectors and Matrices • X: m-by-n real matrix. • Xi:ith column of X. • C: Subset of columns of X. • XC: sub-matrix of X on C. SODA 2012: Ali Kemal Sinop

  4. Formal Problem Definition • Given m-by-n matrix X = [X1 X2 ... Xn], • Find r columns, C, which minimizes: • which is equal to: Projection distance of Xi to XC. SODA 2012: Ali Kemal Sinop

  5. What is Distance to Span? • Given any matrix A, and vector x, • (Pythagorean Theorem) • Thus Orthonormal projection matrix onto null space of A. SODA 2012: Ali Kemal Sinop

  6. Back to Formal Problem Definition • Given m-by-n matrix X = [X1 X2 ... Xn], • Find r columns, C, which minimizes: • No books, • No web pages, • No images. • Only geometry. SODA 2012: Ali Kemal Sinop

  7. Problem Formulation • Given m-by-n matrix X = [X1 X2 ... Xn], • Find r columns, C, which minimizes: • No books, • No web pages, • No images. • Only geometry. Reconstruction Error = is the orthonormal projection matrix onto null space of XC. SODA 2012: Ali Kemal Sinop

  8. An Example • n=2, m=2, r=1: For C={1}, For C={2}, X1 X2 135o Origin SODA 2012: Ali Kemal Sinop

  9. What is the minimum possible? • X is m-by-n. • XC is m-by-r: • Rank of XC is at most |C|=r. • Replace column restriction with rank restriction: • Choose any matrix X(r) of rank-r • Minimizing |C|≤r implies rank≤r SODA 2012: Ali Kemal Sinop

  10. Low Rank Matrix Approximation • Therefore • X(r): Can be found by Singular Value Decomposition (SVD). X(r): a rank-r matrix minimizing SODA 2012: Ali Kemal Sinop

  11. Singular Values of X • There exists m unique non-negative reals, • Best rank-r reconstruction error: • “Smooth rank” of X. • For example, if rank(X) = k, then SODA 2012: Ali Kemal Sinop

  12. First Example • n=2, m=2, r=1. Remember X1 X2 135o Origin Quick check: Worst Possible? SODA 2012: Ali Kemal Sinop

  13. Our Goal: Do as well as best rank-k • Given target rank k, • Allowed error ε>0, • Choose smallest C:|C|=r, such that • How does r depend on k and ε? Best possible rank-k approximation error. SODA 2012: Ali Kemal Sinop

  14. Practical Motivations • [Drineas, Mahoney’09] DNA microarray: • Unsupervised feature selection for cancer detection. • Column Selection + K-means: Better classification. • Many classification problems • Same idea. SODA 2012: Ali Kemal Sinop

  15. Theoretical Applications • Our motivation. • [Guruswami, S’11] Approximation schemes for many graph partitioning problems. • Running time: Exponential in r=r(k,ε) where k=number of eigenvalues < 1-ε. • [Guruswami, S’12] Significantly faster algorithm for sparsest cut and etc... • Running time: Exponential in r=r(k, ε) where k=number of eigenvalues < Φ/ε. r = Number of columns needed to get within (1+ ε) factor of best rank-k approximation SODA 2012: Ali Kemal Sinop

  16. Previous Results • [Frieze, Kannan, Vempala’04] Introduced this problem. • [Deshpande, Vempala’06] • [Sarlos’06] • [Deshpande, Rademacher’10] r=k when ε=k+1. SODA 2012: Ali Kemal Sinop

  17. Recent Results r is optimal (up to low order terms). • [This paper] We showed • r=k/ε+k-1 columns suffice • and r ≥ k/ε-o(k) necessary. • A randomized algorithm in time , • A deterministic algorithm in time • Using [Deshpande, Rademacher’10]. ω=matrix multiplication. • (Independently) [Boutsidis, Drineas, Magdon-Ismail’11] • r≤2 k / ε columns, • In randomized time O(knm/ε + k3ε-2/3n) SODA 2012: Ali Kemal Sinop

  18. Outline • Introduction • Upper Bound • Strategy • An Algebraic Expression • Eliminating Min • Wrapping Up • Randomized Algorithm • Summary SODA 2012: Ali Kemal Sinop

  19. Upper Bound • Input: m-by-n matrix X, target rank k, number of columns r. • Problem: Relate to • Our Approach: • Represent in an algebraic form. • Eliminate minimum by randomly sampling C. • Represent error as a function of σ’s. • Bound it in terms of Best possible rank-k approximation error. SODA 2012: Ali Kemal Sinop

  20. An Algebraic Expression • Remember, our problem is: • is hard to manipulate. • An equivalent algebraic expression? SODA 2012: Ali Kemal Sinop

  21. Base Case: r=1 • A simple case. When C={c}: Xc Xi SODA 2012: Ali Kemal Sinop

  22. Case of r=2 • Consider C={c,d} in 3-dimensions: Xc Xd Xi SODA 2012: Ali Kemal Sinop

  23. General Case • Fact: Volume2 = determinant. • Using Volume = Base-Volume * Height formula, • Hence SODA 2012: Ali Kemal Sinop

  24. Eliminating Min • Volume Sampling[Deshpande, Rademacher, Vempala, Wang’06] • Choose C with probability SODA 2012: Ali Kemal Sinop

  25. Symmetric Forms • Fact: For any k, kth elementary symmetric polynomial: SODA 2012: Ali Kemal Sinop

  26. Schur Concavity • Hence • This ratio is Schur-concave. In other words: ... ... < < ... ... σ1 σ1 σ1 σ2 σ2 σ2 σk σk σk σk+1 σk+1 σk+1 σm σm σm ... ... SODA 2012: Ali Kemal Sinop

  27. Wrapping Up QED For r=k/ε+k-1, this is (1+ε). SODA 2012: Ali Kemal Sinop

  28. Algorithms for Choosing C • (Main idea) A nice recursion: • Randomized Algorithm • Choose j wp • Can be done in time • For all i, • Choose r-1 columns on these vectors. SODA 2012: Ali Kemal Sinop

  29. Outline • Introduction • Upper Bound • Randomized Algorithm • Summary SODA 2012: Ali Kemal Sinop

  30. Summary • (Upper Bound) r=k/ε+k-1 columns suffice to achieve (1+ε)*best rank-k error. • (Randomized) Such columns can be found in time (r TSVD) = O(k) TSVD. • (Lower Bound) k/ε-o(k) columns needed. Thanks! Job market alert. SODA 2012: Ali Kemal Sinop

  31. SODA 2012: Ali Kemal Sinop