Please cite the multcomp package by the following reference:

Torsten Hothorn, Frank Bretz and Peter Westfall (2008). Simultaneous Inference in General Parametric Models. Biometrical Journal 50(3), 346--363.

Corresponding BibTeX entry:

  @Article{,
    title = {Simultaneous Inference in General Parametric Models},
    author = {Torsten Hothorn and Frank Bretz and Peter Westfall},
    journal = {Biometrical Journal},
    year = {2008},
    volume = {50},
    number = {3},
    pages = {346--363},
  }