Two common approaches in low-rank optimization problems are either working
directly with a rank constraint on the matrix variable, or optimizing over a
low-rank factorization so that the rank constraint is implicitly ensured. In
this paper, we study the natural connection between the rank-constrained and
factorized approaches. We show that all second-order stationary points of the
factorized objective function correspond to fixed points of projected gradient
descent run on the original problem (where the projection step enforces the
rank constraint). This result allows us to unify many existing optimization
guarantees that have been proved specifically in either the rank-constrained or
the factorized setting, and leads to new results for certain settings of the
problem. We demonstrate application of our results to several concrete low-rank
optimization problems arising in matrix inverse problems.

本文研究了低秩优化问题中矩阵变量秩约束和低秩分解优化方法的自然联系，证明了低秩分解目标函数的所有二阶稳定点对应于原问题上投影梯度下降算法的不动点，并统一了现有的优化保证，为某些情况下特定的问题提供了新结果。最后，将结果应用于矩阵逆问题。