@misc{indiciae7f915093b98e,
  title = {Closed Surfaces Without Conjugate Points.},
  author = {E Hopf},
  year = {1948},
  doi = {10.1073/pnas.34.2.47},
  url = {https://pubmed.ncbi.nlm.nih.gov/16588785/},
  note = {Source identifier: 16588785}
}