To cite Formula in publications use

Achim Zeileis, Yves Croissant (2010). Extended Model Formulas in R: Multiple Parts and Multiple Responses. Journal of Statistical Software 34(1), 1-13. URL http://www.jstatsoft.org/v34/i01/.

Corresponding BibTeX entry:

  @Article{,
    title = {Extended Model Formulas in {R}: Multiple Parts and
      Multiple Responses},
    author = {Achim Zeileis and Yves Croissant},
    journal = {Journal of Statistical Software},
    year = {2010},
    volume = {34},
    number = {1},
    pages = {1--13},
    url = {http://www.jstatsoft.org/v34/i01/},
  }