@article{courant1967,
  title={On the Partial Difference Equations of Mathematical Physics},
  author={Richard Courant and Kurt Friedrichs and Hans Lewy},
  year={1967},
  journal={IBM Journal of Research and Development},
  doi={10.1147/rd.112.0215},
  url={https://doi.org/10.1147/rd.112.0215}
}