@article{HellwigVolkmann2004,
  author    = {Hellwig, Angelika and Volkmann, Lutz},
  title     = {Neighborhood and degree conditions for super-edge-connected bipartite digraphs},
  series = {Results in mathematics},
  volume    = {Vol. 45},
  journal   = {Results in mathematics},
  number    = {Iss. 1-2},
  issn      = {1420-9012 (E-Journal); 1422-6383 (Print)},
  pages     = {45 -- 58},
  year      = {2004},
  language  = {en}
}
@article{HellwigVolkmann2004,
  author    = {Hellwig, Angelika and Volkmann, Lutz},
  title     = {Sufficient conditions for λ′-optimality in graphs of diameter 2},
  series = {Discrete mathematics},
  volume    = {Vol. 283},
  journal   = {Discrete mathematics},
  number    = {Iss. 1-3},
  issn      = {1872-681X (E-Journal); 0012-365X (Print)},
  pages     = {113 -- 120},
  year      = {2004},
  language  = {en}
}