To cite Formula in publications use:

Zeileis A, Croissant Y (2010). “Extended Model Formulas in R: Multiple Parts and Multiple Responses.” Journal of Statistical Software, 34(1), 1–13. doi:10.18637/jss.v034.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},
    doi = {10.18637/jss.v034.i01},
  }