To cite Formula in publications use

Yves Croissant, Achim Zeileis (2009). Formula: Extended Formulas. R package version 0.2-0. URL http://CRAN.R-project.org/package=Formula

Corresponding BibTeX entry:

  @Manual{,
    title = {{Formula}: Extended Formulas},
    author = {Yves Croissant and Achim Zeileis},
    year = {2009},
    note = {R package version 0.2-0},
    url = {http://CRAN.R-project.org/package=Formula},
  }