@article{JMLR:v22:20-610,
  author  = {Tony Cai and Hongzhe Li and Rong Ma},
  title   = {Optimal Structured Principal Subspace Estimation: Metric Entropy and Minimax Rates},
  journal = {Journal of Machine Learning Research},
  year    = {2021},
  volume  = {22},
  number  = {46},
  pages   = {1--45},
  url     = {http://jmlr.org/papers/v22/20-610.html}
}