nips nips2013 nips2013-11 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Bernardino Romera-Paredes, Massimiliano Pontil
Abstract: We study the problem of learning a tensor from a set of linear measurements. A prominent methodology for this problem is based on a generalization of trace norm regularization, which has been used extensively for learning low rank matrices, to the tensor setting. In this paper, we highlight some limitations of this approach and propose an alternative convex relaxation on the Euclidean ball. We then describe a technique to solve the associated regularization problem, which builds upon the alternating direction method of multipliers. Experiments on one synthetic dataset and two real datasets indicate that the proposed method improves significantly over tensor trace norm regularization in terms of estimation error, while remaining computationally tractable. 1
Reference: text
sentIndex sentText sentNum sentScore
1 uk Abstract We study the problem of learning a tensor from a set of linear measurements. [sent-9, score-0.669]
2 A prominent methodology for this problem is based on a generalization of trace norm regularization, which has been used extensively for learning low rank matrices, to the tensor setting. [sent-10, score-1.232]
3 In this paper, we highlight some limitations of this approach and propose an alternative convex relaxation on the Euclidean ball. [sent-11, score-0.232]
4 We then describe a technique to solve the associated regularization problem, which builds upon the alternating direction method of multipliers. [sent-12, score-0.169]
5 Experiments on one synthetic dataset and two real datasets indicate that the proposed method improves significantly over tensor trace norm regularization in terms of estimation error, while remaining computationally tractable. [sent-13, score-1.269]
6 1 Introduction During the recent years, there has been a growing interest on the problem of learning a tensor from a set of linear measurements, such as a subset of its entries, see [9, 17, 22, 23, 25, 26, 27] and references therein. [sent-14, score-0.669]
7 This methodology, which is also referred to as tensor completion, has been applied to various fields, ranging from collaborative filtering [15], to computer vision [17], and medical imaging [9], among others. [sent-15, score-0.669]
8 In this paper, we propose a new method to tensor completion, which is based on a convex regularizer which encourages low rank tensors and develop an algorithm for solving the associated regularization problem. [sent-16, score-1.275]
9 Arguably the most widely used convex approach to tensor completion is based upon the extension of trace norm regularization [24] to that context. [sent-17, score-1.482]
10 This involves computing the average of the trace norm of each matricization of the tensor [16]. [sent-18, score-1.252]
11 A key insight behind using trace norm regularization for matrix completion is that this norm provides a tight convex relaxation of the rank of a matrix defined on the spectral unit ball [8]. [sent-19, score-1.366]
12 Unfortunately, the extension of this methodology to the more general tensor setting presents some difficulties. [sent-20, score-0.693]
13 In particular, we shall prove in this paper that the tensor trace norm is not a tight convex relaxation of the tensor rank. [sent-21, score-2.066]
14 The above negative result stems from the fact that the spectral norm, used to compute the convex relaxation for the trace norm, is not an invariant property of the matricization of a tensor. [sent-22, score-0.736]
15 This observation leads us to take a different route and study afresh the convex relaxation of tensor rank on the Euclidean ball. [sent-23, score-0.974]
16 We show that this relaxation is tighter than the tensor trace norm, and we describe a technique to solve the associated regularization problem. [sent-24, score-1.172]
17 This method builds upon the alternating direction method of multipliers and a subgradient method to compute the proximity operator of the proposed regularizer. [sent-25, score-0.354]
18 Furthermore, we present numerical experiments on one synthetic dataset and two real-life datasets, which indicate that the proposed method improves significantly over tensor trace norm regularization in terms of estimation error, while remaining computationally tractable. [sent-26, score-1.289]
19 In Section 2, we describe the tensor completion framework. [sent-28, score-0.854]
20 In Section 3, we highlight some limitations of the tensor trace norm regularizer and present an alternative convex relaxation for the tensor rank. [sent-29, score-2.171]
21 An N -order tensor W ∈ Rp1 ×···×pN , is a collection of real numbers (Wi1 ,. [sent-42, score-0.669]
22 W, will be used to denote tensors of order higher than two. [sent-48, score-0.19]
23 Vectors are 1-order tensors and will be denoted by lower case letters, e. [sent-49, score-0.19]
24 x or a; matrices are 2-order tensors and will be denoted by upper case letters, e. [sent-51, score-0.19]
25 A mode-n fiber of a tensor W is a vector composed of the elements of W obtained by fixing all indices but one, corresponding to the n-th mode. [sent-62, score-0.725]
26 The mode-n matricization (or unfolding) of W, denoted by W(n) , is a matrix obtained by arranging the mode-n fibers of W so that each of them is a column of W(n) ∈ Rpn ×Jn , where Jn := k=n pk . [sent-64, score-0.139]
27 We choose a linear operator I : Rp1 ×···×pN → Rm , representing a set of linear measurements obtained from a target tensor W 0 as y = I(W 0 )+ξ, where ξ is some disturbance noise. [sent-67, score-0.721]
28 Tensor completion is an important example of this setting, in this case the operator I returns the known elements of the tensor. [sent-68, score-0.242]
29 Our aim is to recover the tensor W 0 from the data (I, y). [sent-73, score-0.669]
30 The role of the regularizer R is to encourage solutions W which have a simple structure in the sense that they involve a small number of “degrees of freedom”. [sent-75, score-0.135]
31 Specifically, we consider the combinatorial regularizer R(W) = 1 N N rank(W(n) ). [sent-77, score-0.135]
32 (2) n=1 Finding a convex relaxation of this regularizer has been the subject of recent works [9, 17, 23]. [sent-78, score-0.367]
33 This is defined as the average of the trace norm of each matricization of W, that is, W tr = 1 N N W(n) tr (3) n=1 where W(n) tr is the trace (or nuclear) norm of matrix W(n) , namely the ℓ1 -norm of the vector of singular values of matrix W(n) (see, e. [sent-80, score-1.343]
34 Note that in the particular case of 2-order tensors, functions (2) and (3) coincide with the usual notion of rank and trace norm of a matrix, respectively. [sent-83, score-0.539]
35 A rational behind the regularizer (3) is that the trace norm is the tightest convex lower bound to the rank of a matrix on the spectral unit ball, see [8, Thm. [sent-84, score-0.901]
36 This lower bound is given by the convex envelope of the function rank(W ), if W ∞ ≤ 1 (4) Ψ(W ) = +∞, otherwise 1 For simplicity we assume that pn ≥ 2 for every n ∈ [N ], otherwise we simply reduce the order of the tensor without loss of information. [sent-86, score-1.075]
37 The convex envelope can be derived by computing the double conjugate of Ψ. [sent-88, score-0.246]
38 Note that Ψ is a spectral function, that is, Ψ(W ) = ψ(σ(W )) where ψ : Rd → R denotes the + associated symmetric gauge function. [sent-90, score-0.141]
39 3 Alternative Convex Relaxation In this section, we show that the tensor trace norm is not a tight convex relaxation of the tensor rank R in equation (2). [sent-97, score-2.164]
40 We then propose an alternative convex relaxation for this function. [sent-98, score-0.232]
41 Note that due to the composite nature of the function R, computing its convex envelope is a challenging task and one needs to resort to approximations. [sent-99, score-0.255]
42 In [22], the authors note that the tensor trace norm · tr in equation (3) is a convex lower bound to R on the set G∞ := W ∈ Rp1 ×···×pN : W(n) ∞ ≤ 1, ∀n ∈ [N ] . [sent-100, score-1.355]
43 However, the authors of [22] leave open the question of whether the tensor trace norm is the convex envelope of R on the set G∞ . [sent-102, score-1.36]
44 In the following, we will prove that this question has a negative answer by showing that there exists a convex function Ω = · tr which underestimates the function R on G∞ and such that for some tensor W ∈ G∞ it holds that Ω(W) > W tr . [sent-103, score-0.938]
45 ×pN : W where · 2 2 ≤1 is the Euclidean norm for tensors, that is, p1 W 2 2 := i1 =1 pN ··· (Wi1 ,. [sent-107, score-0.186]
46 iN =1 We will choose Ω(W) = Ωα (W) := 1 N N ∗∗ ωα σ W(n) (6) n=1 ∗∗ where ωα is the √ convex envelope of the cardinality of a vector on the ℓ2 -ball of radius α and we will choose α = pmin . [sent-111, score-0.415]
47 Note, by Lemma 4 stated in Appendix A, that for every α > 0, function Ωα is a convex lower bound of function R on the set αG2 . [sent-112, score-0.142]
48 Let ωα be the convex envelope of the cardinality on the ℓ2 -ball of radius α. [sent-115, score-0.264]
49 The function ωα resembles the norm developed in [1], which corresponds to the convex envelope of the indicator function of the cardinality of a vector in the ℓ2 ball. [sent-118, score-0.45]
50 The extension of its application to tensors is not straighforward though, as it is required to specify beforehand the rank of each matricization. [sent-119, score-0.263]
51 The next lemma provides, together with Lemma 1, a sufficient condition for the existence of a tensor W ∈ G∞ at which the regularizer in equation (6) is strictly larger than the tensor trace norm. [sent-120, score-1.813]
52 , pN are√not all equal to each other, then there exists W ∈ Rp1 ×···×pN such that: (a) W 2 = pmin , (b) W ∈ G∞ , (c) min rank(W(n) ) < n∈[N ] max rank(W(n) ). [sent-125, score-0.151]
53 , pN ∈ N, let · tr be the tensor trace norm in equation (3) and let √ Ωα be the function in equation (6) for α = pmin . [sent-132, score-1.41]
54 If pmin < pmax , then there are infinitely many tensors W ∈ G∞ such that Ωα (W) > W tr . [sent-133, score-0.451]
55 Since G∞ ⊂ αG2 then Ωα is a convex lower bound for the tensor rank R on the set G∞ as well. [sent-137, score-0.863]
56 Indeed, all tensors obtained following the process described in the proof of Lemma 2 (in Appendix C) have the property that tr N = 1 N < W 1 (pmin (N − 1) + pmin + 1) = Ω(W) = R(W). [sent-139, score-0.415]
57 N σ(W(n) ) 1 = n=1 1 N pmin (N − 1) + p2 + pmin min Furthermore there are infinitely many such tensors which satisfy this claim (see Appendix C). [sent-140, score-0.519]
58 ∗∗ With respect to the second claim, given that ω1 is the convex envelope of the cardinality card on ∗∗ the Euclidean unit ball, then ω1 (σ) ≥ σ 1 for every vector σ such that σ 2 ≤ 1. [sent-141, score-0.317]
59 The above result stems from the fact that the spectral norm is not an invariant property of the matricization of a tensor, whereas the Euclidean (Frobenius) norm is. [sent-143, score-0.596]
60 4 Optimization Method In this section, we explain how to solve the regularization problem associated with the regularizer (6). [sent-145, score-0.222]
61 For this purpose, we first recall the alternating direction method of multipliers (ADMM) [4], which was conveniently applied to tensor trace norm regularization in [9, 22]. [sent-146, score-1.3]
62 In particular, if ψ is the ℓ1 norm then problem (7) corresponds to tensor trace norm regular∗∗ ization, whereas if ψ = ωα it implements the proposed regularizer. [sent-150, score-1.341]
63 By completing the square in the right hand side, the solution of this problem is given by ˆ 1 Bn(n) = prox β Ψ (X) := argmin Bn(n) 1 1 Bn(n) − X Ψ Bn(n) + β 2 2 2 , 1 where X = W(n) − β An(n) . [sent-169, score-0.161]
64 1]) we know that if ψ is a gauge function then ⊤ 1 1 prox β Ψ (X) = UX diag prox β ψ (σ(X)) VX , where UX and VX are the orthogonal matrices formed by the left and right singular vectors of X, respectively. [sent-174, score-0.321]
65 If we choose ψ = · 1 the associated proximity operator is the well-known soft 1 thresholding operator, that is, prox β · 1 (σ) = v, where the vector v has components vi = sign (σi ) |σi | − 1 β . [sent-175, score-0.303]
66 2 Computation of the Proximity Operator 1 ∗∗ To compute the proximity operator of the function β ωα we will use several properties of proximity 1 ∗∗ calculus. [sent-179, score-0.276]
67 Finally, 1 by the scaling property of proximity operators [7], we have that proxg (x) = β proxβωα (βx). [sent-184, score-0.17]
68 ∗ 2 The somewhat cumbersome notation Bn(n) denotes the mode-n matricization of tensor Bn , that is, Bn(n) = (Bn )(n) . [sent-185, score-0.786]
69 5 Experiments We have conducted a set of experiments to assess whether there is any advantage of using the proposed regularizer over the tensor trace norm for tensor completion3 . [sent-204, score-1.99]
70 Then, we have tried both methods on two tensor completion real data problems. [sent-206, score-0.856]
71 It should take the value of the Euclidean norm of the underlying tensor. [sent-212, score-0.186]
72 This estimator assumes that each value in the tensor is sampled from N (mean(w), var(w)), where mean(w) and var(w) are the average and the variance of the elements in w. [sent-214, score-0.699]
73 0085 −5 −4 −3 2 log σ −2 0 −1 50 100 150 200 p Figure 1: Synthetic dataset: (Left) Root Mean Squared Error (RMSE) of tensor trace norm and the proposed regularizer. [sent-223, score-1.155]
74 1 Synthetic Dataset We have generated a 3-order tensor W 0 ∈ R40×20×10 by the following procedure. [sent-226, score-0.669]
75 First we generated a tensor W with ranks (12, 6, 3) using Tucker decomposition (see e. [sent-227, score-0.689]
76 We then created the ground truth tensor W 0 by the equation 0 Wi1 ,i2 ,i3 = Wi1 ,i2 ,i3 − mean(W) √ + ξi1 ,i2 ,i3 N std(W) where mean(W) and std(W) are the mean and standard deviation of the elements of W, N is the total number of elements of W, and the ξi1 ,i2 ,i3 are i. [sent-230, score-0.754]
77 We have randomly sampled 10% of the elements of the tensor to compose the training set, 45% for the validation set, and the remaining 45% for the test set. [sent-234, score-0.747]
78 We have generated tensors W 0 ∈ Rp×p×p for different values of p ∈ {20, 40, . [sent-239, score-0.19]
79 For low values of p, the ratio between the running time of our approach and that of the trace norm regularization method is quite high. [sent-244, score-0.532]
80 However, as the volume of the tensor increases, the ratio quickly decreases. [sent-247, score-0.669]
81 Therefore, we can conclude that even though our approach is slower than the trace norm based method, this difference becomes much smaller as the size of the tensor increases. [sent-252, score-1.135]
82 It is composed of examination marks ranging from 0 to 70, of 15362 students who are described by a set of attributes such as school and ethnic group. [sent-255, score-0.122]
83 Most of these attributes are categorical, thereby we can think of exam mark prediction as a tensor completion problem where each of the modes corresponds to a categorical attribute. [sent-256, score-0.877]
84 In particular, we have used the following attributes: school (139), gender (2), VR-band (3), ethnic (11), and year (3), leading to a 5-order tensor W ∈ R139×2×3×11×3 . [sent-257, score-0.739]
85 4 26 24 4000 6000 8000 10000 m (Training Set Size) 12000 2 4 6 8 10 12 m (Training Set Size) 14 16 4 x 10 Figure 2: Root Mean Squared Error (RMSE) of tensor trace norm and the proposed regularizer for ILEA dataset (Left) and Ocean video (Right). [sent-264, score-1.353]
86 There is a distinguishable improvement of our approach with respect to tensor trace norm regularization for values of m > 7000. [sent-268, score-1.201]
87 3 Video Completion In the second real-data experiment we have performed a video completion test. [sent-273, score-0.223]
88 Any video can be treated as a 4-order tensor: “width” × “height” × “RGB” × “video length”, so we can use tensor completion algorithms to rebuild a video from a few inputs, a procedure that can be useful for compression purposes. [sent-274, score-0.955]
89 This video sequence can be treated as a tensor W ∈ R160×112×3×32 . [sent-276, score-0.732]
90 We have randomly sampled m tensors elements as training data, 5% of them as validation data, and the remaining ones composed the test set. [sent-277, score-0.294]
91 The proposed approach is noticeably better than the tensor trace norm in this experiment. [sent-279, score-1.155]
92 6 Conclusion In this paper, we proposed a convex relaxation for the average of the rank of the matricizations of a tensor. [sent-282, score-0.364]
93 We compared this relaxation to a commonly used convex relaxation used in the context of tensor completion, which is based on the trace norm. [sent-283, score-1.292]
94 We proved that this second relaxation is not tight and argued that the proposed convex regularizer may be advantageous. [sent-284, score-0.417]
95 Our numerical experience indicates that our method consistently improves in terms of estimation error over tensor trace norm regularization, while being computationally comparable on the range of problems we considered. [sent-285, score-1.176]
96 In the future it would be interesting to study methods to speed up the computation of the proximity operator of our regularizer and investigate its utility in tensor learning problems beyond tensor completion such as multilinear multitask learning [20]. [sent-286, score-1.848]
97 Tensor completion and low-n-rank tensor recovery via convex optimization. [sent-352, score-0.95]
98 Multiverse recommendation: n-dimensional tensor factorization for context-aware collaborative filtering. [sent-392, score-0.669]
99 Tensor completion for estimating missing values in visual data. [sent-407, score-0.16]
100 Learning with tensors: a framework based on convex optimization and spectral regularization. [sent-438, score-0.184]
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