Paper:
Gradient Descent Meets Shift-and-Invert Preconditioning for Eigenvector Computation
Shift-and-invert preconditioning, as a classic acceleration technique for the leading eigenvector computation, has received much attention again recently, owing to fast least-squares solvers for efficiently approximating matrix inversions in power iterations. In this work, we adopt an inexact Riemannian gradient descent perspective to investigate this technique on the effect of the step-size scheme. The shift-and-inverted power method is included as a special case with adaptive step-sizes. Particularly, two other step-size settings, i.e., constant step-sizes and Barzilai-Borwein (BB) step-sizes, are examined theoretically and/or empirically. We present a novel convergence analysis for the constant step-size setting that achieves a rate atÕ(
, where λ i represents the i-th largest eigenvalue of the given real symmetric matrix and p is the multiplicity of λ 1 . Our experimental studies show that the proposed algorithm can be significantly faster than the shift-and-inverted power method in practice.

Review:
This paper proposes an algorithm which combines shift-and-invert preconditioning and gradient descent to compute leading eigenvector. Shift-and-invert preconditioning is a common preconditioning technique, and gradient descent is one of the most popular optimization algorithms, so this combination seems reasonable. They establish the complexity of using this method, by using a somewhat new potential function.  Pros: The main difference of this paper with previous works is that it allows the multiplicity of the leading eigenvalue to be larger than 1, while most works assume an eigen-gap between the largest and second largest eigenvalue.          In addition, this paper introduces a somewhat new potential function, that depends on the angle between a vector and the subspace spanned by the eigenvectors. More specifically, the proof uses two potential functions, one is log(u) and another is 1 – sin^2(v), where u and v depend on the angle. The second one is not new, but the first one log(.) seems to be new.    Cons: There are quite a few issues to be addressed/clarified.  1.  Complexity.   Perhaps the biggest issue is: the complexity proved in this work is not shown to be better than the other existing results (a bit subtle; see explanation below). In fact, we need to discuss two cases:   Case 1: the multiplicity of the largest eigenvalue is 1, i.e $\lambda_1 > \lambda_2 = \lambda_{p+1}$. In this case, the complexity of this work is not better than those in [Musco and Musco, 2015] and [Garber et al., 2016] (in fact, the same).   Case 2: the multiplicity of the leading eigenvalue is larger than 1. In this case, the proposed rate is the “best“ so far. However, most other works do not consider this scenario, so this is not a fair comparison.   Overall, the improvement only in Case 2 seems to be a minor progress.   BTW: in Table 1, it seems that $\sqrt{\lambda_1}$ is missing from the numerator of the rate for ``this work''.   2. Limited Novelty of Proposed Algorithm    There are already a few works which combine shift-and-invert preconditioning with some common base methods, such as accelerated gradient descent [Wang et al., 2017], accelerated randomized coordinate descent methods [Wang et al., 2017] or SVRG [Garber et al., 2016]. It is not surprising to use gradient descent (GD) for subproblems. From the arguments made in paper, I fail to see why GD will be a better choice.          First, the authors claim that GD is better because Lanczos algorithms does not work for positive definite matrix, but GD does (line 39). However, some other base methods, such as``coordinate-wise'' methods [Wang et al., 2017], also work for positive definite matrices.         Second, the authors claim GD will not be stuck at saddle points with probability one from a random starting point. The authors make this point by citing [Pitaval et al., 2015] (line 42). However, [Pitaval et al., 2015] is about a quite different problem. In addition, people tend to believe all reasonable first order methods will not be stuck at saddle points.          The warm-start technique (line 158) is also not new to the community. The same formulation of warm-start has been used in [Garber et al., 2016].  3 Algorithm Details         The first negative sign in line 4 of Algorithm 1 should be removed. From equation (3), Riemannian gradient is equal to Euclidean gradient left multiplied by $(I - x_{t-1} x_{t-1}^T)$. In Algorithm 1, $y_t$ is the approximate Euclidean gradient. Therefore, the approximation Riemannian gradient in line 4 should be $(I - x_{t-1} x_{t-1}^T) y_t$.         Another thing that requires clarification is the choice of stepsize. In Algorithm 1, authors pick the stepsize at every iteration such that the stepsize satisfies the second inequality in line 176. Why do the authors pick the stepsize at each iteration $t$, while the second inequality in line 176 is not dependent on each iteration $t$. Why don't we pick a constant stepsize before running the iterations?   4. Experiments The authors only compare Rimennian gradient descent solver with/without shift-and-invert preconditioning, and shift-and-inverted power method. As I mentioned before, the authors does not show the complexity of this work is better than [Musco and Musco, 2015] (Block Krylov) and [Garber et al., 2016] (shift-and-inverted power method). Therefore, it is reasonable to also consider comparing with these methods.  5. Minor issues:   --It seems that the authors did not explicitly define Riemannian gradient.    -- line 171 should be Lemma 3.2 rather than 3.3.  After the rebuttal: I've increased my score by one, due to the good empirical performance. The tone of this paper is like "our contribution is to combine two methods, and get the best rate so far", and my above comments said, "combining two methods is rather straightforward, and the rate does not improve previous ones" --thus a clear reject. If the authors motivate the paper from an empirical perspective by saying "we find the combination of SI+GD works really well, so to promote it we provide theoretical justification", the contribution may be more clear. Considering the above reasons, I think this paper is slightly below the acceptance threshold.  [Garber et al., 2016] Dan Garber, Elad Hazan, Chi Jin, Sham M. Kakade, Cameron Musco, Praneeth Netrapalli, and Aaron Sidford. Faster eigenvector computation via shift-and-invert preconditioning. In International Conference on Machine Learning, pages 2626–2634, 2016.  [Musco and Musco, 2015] Cameron Musco and Christopher Musco. Randomized block krylov methods for stronger and faster approximate singular value decomposition. In Advances in Neural Information Processing Systems, pages 1396–1404, 2015.  [Pitaval et al., 2015] Renaud-Alexandre Pitaval, Wei Dai, and Olav Tirkkonen. Convergence of gradient descent for low-rank matrix approximation. IEEE Trans. Information Theory, 61(8):4451–4457, 2015.  [Wang et al., 2017] Jialei Wang, Weiran Wang, Dan Garber, and Nathan Srebro. Efficient coordinate-wise leading eigenvector computation. CoRR, abs/1702.07834, 2017. URL http://arxiv.org/abs/1702. 07834.