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1 Algorithms for Large Data Sets Ziv Bar-Yossef Lecture 4 March 30, 2005 http://www.ee.technion.ac.il/courses/049011

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2 Spectral Methods in Information Retrieval

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3 Outline Motivation: synonymy and polysemy Latent Semantic Indexing (LSI) Singular Value Decomposition (SVD) LSI via SVD Why LSI works? HITS and SVD

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4 Synonymy and Polysemy Synonymy: multiple terms with (almost) the same meaning Ex: cars, autos, vehicles Harms recall Polysemy: a term with multiple meanings Ex: java (programming language, coffee, island) Harms precision

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5 Traditional Solutions Query expansion Synonymy: OR on all synonyms Manual/automatic use of thesauri Too few synonyms: recall still low Too many synonyms: harms precision Polysemy: AND on term and additional specializing terms Ex: +java +”programming language” Too broad terms: precision still low Too narrow terms: harms recall

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6 Syntactic Space D: document collection, |D| = n T: term space, |T| = m A t,d : “weight” of t in d (e.g., TFIDF) A T A: pairwise document similarities AA T : pairwise term similarities A m n terms documents

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7 Syntactic Indexing Index keys: terms Limitations Synonymy (Near)-identical rows Polysemy Space inefficiency Matrix usually is not full rank Gap between syntax and semantics: Information need is semantic but index and query are syntactic.

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8 Semantic Space C: concept space, |C| = r B c,d : “weight” of c in d Change of basis Compare to wavelet and Fourier transforms B r n concepts documents

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9 Latent Semantic Indexing (LSI) [Deerwester et al. 1990] Index keys: concepts Documents & query: mixtures of concepts Given a query, finds the most similar documents Bridges the syntax-semantics gap Space-efficient Concepts are orthogonal Matrix is full rank Questions What is the concept space? What is the transformation from the syntax space to the semantic space? How to filter “noise concepts”?

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10 Singular Values A: m×n real matrix Definition: ≥ 0 is a singular value of A if there exist a pair of vectors u,v s.t. Av = u and A T u = v u and v are called singular vectors. Ex: = ||A|| 2 = max ||x|| 2 = 1 ||Ax|| 2. Corresponding singular vectors: x that maximizes ||Ax|| 2 and y = Ax / ||A|| 2. Note: A T Av = 2 v and AA T u = 2 u 2 is eigenvalue of A T A and AA T u eigenvector of A T A v eigenvector of AA T

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11 Singular Value Decomposition (SVD) Theorem: For every m×n real matrix A, there exists a singular value decomposition: A = U V T 1 ≥ … ≥ p ≥ 0 (p = min(m,n)): singular values of A = Diag( 1,…, p ) U: column-orthogonal m×m matrix (U T U = I ) V: column-orthogonal m×m matrix (V T V = I ) AU VTVT ×× =

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12 Singular Values vs. Eigenvalues A = U V T 1,…, p : singular values of A 1 2,…, p 2 : eigenvalues of A T A and AA T u 1,…,u m : columns of U Orthonormal basis of R m Left singular vectors of A Eigenvectors of A T A v 1,…,v n : columns of V Orthonormal basis of R n Right singular vectors Eigenvectors of AA T

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13 Economy SVD Let r = max i s.t. i > 0 r+1 = … = p = 0 rank(A) = r u 1,…,u r : left singular vectors v 1,…,v n : right singular vectors U T A = V T AU VTVT ×× = r mm nn r r

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14 LSI as SVD U T A = V T u 1,…,u r : concept basis B = V T : LSI matrix A d : d-th column of A B d : d-th column of B B d = U T A d B d [c] =

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15 Noisy Concepts B = U T A = V T B d [c] = c v d [c] If c is small, then B d [c] small for all d k = largest i s.t. i is “large” For all c = k+1,…,r, and for all d, c is a low- weight concept in d Main idea: filter out all concepts c = k+1,…,r Space efficient: # of index terms = k (vs. r or m) Better retrieval: noisy concepts are filtered out across the board

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16 Low-rank SVD B = U T A = V T U k = (u 1,…,u k ) V k = (v 1,…,v k ) k = upper-left k×k sub-matrix of A k = U k k V k T B k = k V k T rank(A k ) = rank(B k ) = k

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17 Low Dimensional Embedding Forbenius norm: Fact: Therefore, if is small, then for “most” d,d’,. A k preserves pairwise similarities among documents at least as good as A for retrieval.

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18 Why is LSI Better? [Papadimitriou et al. 1998] [Azar et al. 2001] LSI summary Documents are embedded in low dimensional space (m k) Pairwise similarities are preserved More space-efficient But why is retrieval better? Synonymy Polysemy

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19 Generative Model T: term space, |T| = m A concept c: a distribution on T C: concept space, |C| = k C’: space of all convex combinations of concepts D: distribution on C’×N A corpus model M = (T,C’,D) A document d is generated as follows: Sample (w,n) according to D Repeat n times: Sample a concept c from C according to w Sample a term t from T according to c

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20 Simplifying Assumptions A: m×n term-document matrix, representing n instantiations of the model D c : documents whose topic is the concept c T c : terms in supp(c) Assumptions: Every document has a single topic (C’ = C) For every two concepts c,c’, ||c – c’|| ≥ 1 - The probability of every term under a concept c is at most some constant .

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21 LSI Works Theorem [Papadimitriou et al. 1998] Given the above assumptions, then with high probability, for every two documents d,d’, If d,d’ have the same topic, then If d,d’ have different topics, then

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22 Proof For simplicity, assume = 0 Want to show: (1) if d,d’ on same topic, A d k, A d’ k are in the same direction (2) If d,d’ on different topics, A d k, A d’ k are orthogonal A has non-zeroes only in blocks: B 1,…,B k, where B c : sub-matrix of A with rows in T c and columns in D c A T A is a block diagonal matrix with blocks B T 1 B 1,…, B T k B k (i,j)-th entry of B T c B c : term similarity between i-th and j-th documents on the concept c B T c B c : adjacency matrix of a bipartite (multi-)graph G c on D c

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23 Proof (cont.) G c is a “random” graph First and second eigenvalues of B T c B c are well separated For all c,c’, second eigenvalue of B T c B c is smaller than first eigenvalue of B T c’ B c’ Top k eigenvalues of A T A are the principal eigenvalues of B T c B c for c = 1,…,k Let u 1,…,u k be corresponding eigenvectors For every document d on topic c, A d is orthogonal to all u 1,…,u k, except for u d. A k d is a scalar multiple of u d.

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24 Extensions [Azar et al. 2001] A more general generative model Explain also improved treatment of polysemy

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25 Computing SVD Compute singular values of A, by computing eigenvalues of A T A Compute U,V by computing eigenvectors of A T A and AA T Running time not too good: O(m 2 n + m n 2 ) Not practical for huge corpora Sub-linear time algorithms for estimating A_k [Frieze,Kannan,Vempala 1998]

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26 HITS and SVD A: adjacency matrix of a web (sub-)graph G a: authority vector h: hub vector a is principal eigenvector of A T A h is principal eigenvector of AA T Therefore: a and h give A 1 : the rank-1 SVD of A Generalization: using A k, we can get k authority and hub vectors, corresponding to other topics in G.

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27 End of Lecture 4

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