Sharp Global Guarantees for Nonconvex Low-Rank Matrix Recovery in the Overparameterized Regime

We prove that it is possible for nonconvex low-rank matrix recovery to contain no spurious local minima when the rank of the unknown ground truth $r^{\star}<r$ is strictly less than the search rank $r$, and yet for the claim to be false when $r^{\star}=r$. Under the restricted isometry property (RIP), we prove, for the general overparameterized regime with $r^{\star}\le r$, that an RIP constant of $\delta<1/(1+\sqrt{r^{\star}/r})$ is sufficient for the inexistence of spurious local minima, and that $\delta<1/(1+1/\sqrt{r-r^{\star}+1})$ is necessary due to existence of counterexamples. Without an explicit control over $r^{\star}\le r$, an RIP constant of $\delta<1/2$ is both necessary and sufficient for the exact recovery of a rank-$r$ ground truth. But if the ground truth is known a priori to have $r^{\star}=1$, then the sharp RIP threshold for exact recovery is improved to $\delta<1/(1+1/\sqrt{r})$.