Paper:
Matrix Completion from Fewer Entries: Spectral Detectability and Rank Estimation
The completion of low rank matrices from few entries is a task with many practical applications. We consider here two aspects of this problem: detectability, i.e. the ability to estimate the rank r reliably from the fewest possible random entries, and performance in achieving small reconstruction error. We propose a spectral algorithm for these two tasks called MaCBetH (for Matrix Completion with the Bethe Hessian). The rank is estimated as the number of negative eigenvalues of the Bethe Hessian matrix, and the corresponding eigenvectors are used as initial condition for the minimization of the discrepancy between the estimated matrix and the revealed entries. We analyze the performance in a random matrix setting using results from the statistical mechanics of the Hopfield neural network, and show in particular that MaCBetH efficiently detects the rank r of a large n × m matrix from C(r)r √ nm entries, where C(r) is a constant close to 1. We also evaluate the corresponding root-mean-square error empirically and show that MaCBetH compares favorably to other existing approaches. Matrix completion is the task of inferring the missing entries of a matrix given a subset of known entries. Typically, this is possible because the matrix to be completed has (at least approximately) low rank r. This problem has witnessed a burst of activity, see e.g. [1,2,3], motivated by many applications such as collaborative filtering [1], quantum tomography [4] in physics, or the analysis of a covariance matrix [1]. A commonly studied model for matrix completion assumes the matrix to be exactly low rank, with the known entries chosen uniformly at random and observed without noise. The most widely considered question in this setting is how many entries need to be revealed such that the matrix can be completed exactly in a computationally efficient way [1,3]. While our present paper assumes the same model, the main questions we investigate are different.
The first question we address is detectability: how many random entries do we need to reveal in order to be able to estimate the rank r reliably. This is motivated by the more generic problem of detecting structure (in our case, low rank) hidden in partially observed data. It is reasonable to expect the existence of a region where exact completion is hard or even impossible yet the rank estimation is tractable. A second question we address is what is the minimum achievable root-mean-square error (RMSE) in estimating the unknown elements of the matrix. In practice, even if exact reconstruction is not possible, having a procedure that provides a very small RMSE might be quite sufficient.
In this paper we propose an algorithm called MaCBetH that gives the best known empirical performance for the two tasks above when the rank r is small. The rank in our algorithm is estimated as the number of negative eigenvalues of an associated Bethe Hessian matrix [5,6], and the corresponding eigenvectors are used as an initial condition for the local optimization of a cost function commonly considered in matrix completion (see e.g. [3]). In particular, in the random matrix setting, we show 1
Matrix completion is the task of inferring the missing entries of a matrix given a subset of known entries. Typically, this is possible because the matrix to be completed has (at least approximately) low rank r. This problem has witnessed a burst of activity, see e.g. [1,2,3], motivated by many applications such as collaborative filtering [1], quantum tomography [4] in physics, or the analysis of a covariance matrix [1]. A commonly studied model for matrix completion assumes the matrix to be exactly low rank, with the known entries chosen uniformly at random and observed without noise. The most widely considered question in this setting is how many entries need to be revealed such that the matrix can be completed exactly in a computationally efficient way [1,3]. While our present paper assumes the same model, the main questions we investigate are different.
The first question we address is detectability: how many random entries do we need to reveal in order to be able to estimate the rank r reliably. This is motivated by the more generic problem of detecting structure (in our case, low rank) hidden in partially observed data. It is reasonable to expect the existence of a region where exact completion is hard or even impossible yet the rank estimation is tractable. A second question we address is what is the minimum achievable root-mean-square error (RMSE) in estimating the unknown elements of the matrix. In practice, even if exact reconstruction is not possible, having a procedure that provides a very small RMSE might be quite sufficient.
In this paper we propose an algorithm called MaCBetH that gives the best known empirical performance for the two tasks above when the rank r is small. The rank in our algorithm is estimated as the number of negative eigenvalues of an associated Bethe Hessian matrix [5,6], and the corresponding eigenvectors are used as an initial condition for the local optimization of a cost function commonly considered in matrix completion (see e.g. [3]). In particular, in the random matrix setting, we show that MaCBetH detects the rank of a large n × m matrix from C(r)r √ nm entries, where C(r) is a small constant, see Fig. 2, and C(r) → 1 as r → ∞. The RMSE is evaluated empirically and, in the regime close to C(r)r √ nm, compares very favorably to existing approache such as OptSpace [3].
This paper is organized as follows. We define the problem and present generally our approach in the context of existing work in Sec. 1. In Sec. 2 we describe our algorithm and motivate its construction via a spectral relaxation of the Hopfield model of neural network. Next, in Sec. 3 we show how the performance of the proposed spectral method can be analyzed using, in parts, results from spin glass theory and phase transitions, and rigorous results on the spectral density of large random matrices. Finally, in Sec. 4 we present numerical simulations that demonstrate the efficiency of MaCBetH. Implementations of our algorithms in the Julia and Matlab programming languages are available at the SPHINX webpage http://www.lps.ens.fr/~krzakala/WASP.html.

Review:
This paper proposes a method for estimating the rank of a low-rank incomplete data matrix, based on the so-called Beth free energy, which is an energy function used in statistical mechanics. The authors use the incomplete data matrix to define a graphical model on a bipartite graph, corresponding to the rows and columns of the data matrix. This graphical model is further studied by an approximation called the Beth free energy (BFE), whose phase transition is known in statistical mechanics: as a function of a certain parameter beta, the BFE has a single global optimum for small enough beta. As beta increases a few local minima appear, called the retrieval states. As beta keeps increasing a phase transition occurs in that many spurious local minima appear. The authors use a correspondence between statistical mechanics and the appearence of phase transitions in the graphical model, to estimate the rank of the underlying matrix as the number of retrieval states. This correspondence enables the authors to estimate the minimum number of observed entries per row required to correctly estimate the rank of the matrix. Experiments using synthetic data demonstrate the merit of the proposed method. From reading the paper it is clear that the authors are very knowledgable as far as the Beth approximation is concerned, and their literature review on matrix completion is fair. In addition, their introduction up to the end of page 2 is well written and comprehensive. More importantly, the idea of using the Beth free energy for the purpose of rank-estimation of low-rank incomplete matrices seems original and as the synthetic experiments demonstrate, quite promising. I believe that the right way to judge this paper is as a work that attempts to bring together notions from machine learning (matrix completion) and notions from statistical mechanics (phase transition of Beth free energy). From such a viewpoint the paper fails, as the authors do not clearly establish the correspondence that they are using to translate the physics notions to the machine learning notions. Let me be precise. In the paper, there seems to be a length-2 chain of correspondences: Matrix Completion - Graphical Models - Statistical Mechanics. As far as the first correspondence is concerned, the authors give no motivation or intuition at all at writing down graphical model (6). What does this model mean in terms of matrix completion? Next, from graphical model (6) the authors pass to the Beth free energy (7), for which they claim that its phase transition is well known. But in line 182 they claim without further comment, perhaps the most important claim of this paper, that a good estimate for the rank of the matrix is the number of retrieval states of (7). Where does that come from? I believe the authors should very carefully justify this claim, as this seems to be the backbone of the paper. Next, at lines 194-197 they say that to study the retrieval states they look at the negative eigenvalues of the Hessian of the BFE at its global minimum (paramagnetic state). Again, how is this justified? At lines 266-269 the authors state an important characterization of retrieval states once again without justification. Naturally, i do not expect the authors to reproduce a possibly large amount of results from statistical mechanics so that all arguments are self-contained and rigorous. But i do expect them to tie connections on the high-level, and in the low-level wherever possible, with the problem they are trying to solve, which is estimating the rank and more importantly, they should spend more space on establishing very clearly the correspondences that are being exploited.This paper has the potential of becoming a good paper subject to the authors rewriting it in a more accessible way, and establishing carefully all important connections for their purpose between matrix completion and statistical mechanics.As the paper stands, i can not recommend it for publication, since i believe that many readers would not even understandits basic ideas.