When the linear measurements of an instance of low-rank matrix recovery satisfy a restricted isometry property (RIP)---i.e. they are approximately norm-preserving---the problem is known to contain no spurious local minima, so exact recovery is guaranteed. In this paper, we show that moderate RIP is not enough to eliminate spurious local minima, so existing results can only hold for near-perfect RIP. In fact, counterexamples are ubiquitous: we prove that every x is the spurious local minimum of a rank-1 instance of matrix recovery that satisfies RIP. One specific counterexample has RIP constant $\delta=1/2$, but causes randomly initialized stochastic gradient descent (SGD) to fail 12% of the time. SGD is frequently able to avoid and escape spurious local minima, but this empirical result shows that it can occasionally be defeated by their existence. Hence, while exact recovery guarantees will likely require a proof of no spurious local minima, arguments based solely on norm preservation will only be applicable to a narrow set of nearly-isotropic instances.

本文研究了低秩矩阵恢复的局部极小点问题，分析了一定程度的受限等距性并不能消除这些极小点，而随机梯度下降算法在某些情况下可能无法避免或逃脱这些极小点。因此，对于低秩矩阵恢复的精确恢复保证需要证明不存在这些局部极小点而不是仅仅基于范数的保持。

非凸矩阵恢复需要多少有限制等比性？