@article{10_4171_dm_61,
     author = {Roger T. Lewis and Marcel Griesemer and Heinz Siedentop},
     title = {A minimax principle for eigenvalues in spectral gaps: {Dirac} operators with {Coulomb} potentials.},
     journal = {Documenta mathematica},
     pages = {275--283},
     year = {1999},
     volume = {4},
     doi = {10.4171/dm/61},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/dm/61/}
}