@misc{indiciae5aa9d19a7c1a,
  title = {General flat four-dimensional world pictures and clock systems},
  author = {Hsu, J. P. and Underwood, J. A.},
  year = {1978},
  url = {https://ntrs.nasa.gov/citations/19790066312},
  note = {Source identifier: 19790066312}
}