@article{4418,
  author = {Yves Justum and A. Maquet and Y. Heno},
  title = {Gauge invariance and approximate multiphoton calculations in hydrogen},
  year = {1990},
  journal = {Physical Review A},
  volume = {41},
  pages = {2791-2795},
  month = {Jan-03-1990},
  issn = {1050-2947},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.41.2791},
  doi = {10.1103/PhysRevA.41.2791},
}