Gauge-independent Wigner functions: General formulation.

@article{Serimaa1986GaugeindependentWF,
  title={Gauge-independent Wigner functions: General formulation.},
  author={Serimaa and Javanainen and Varr{\'o}},
  journal={Physical review. A, General physics},
  year={1986},
  volume={33 5},
  pages={2913-2927}
}