@misc{indiciae79717c600259,
  title = {Flat Space-Time and Gravitation.},
  author = {G D Birkhoff},
  year = {1944},
  doi = {10.1073/pnas.30.10.324},
  url = {https://pubmed.ncbi.nlm.nih.gov/16578137/},
  note = {Source identifier: 16578137}
}