To cite the 'cusp' package in publications use:

Raoul P. P. P. Grasman, Han L. J. van der Maas, Eric-Jan Wagenmakers (2009). Fitting the Cusp Catastrophe in R: A cusp Package Primer. Journal of Statistical Software 32(8), 1-27. URL http://www.jstatsoft.org/v32/i08/.

Corresponding BibTeX entry:

  @Article{,
    title = {Fitting the Cusp Catastrophe in {R}: A cusp Package
      Primer},
    author = {Raoul P. P. P. Grasman and Han L. J. van der Maas and
      Eric-Jan Wagenmakers},
    journal = {Journal of Statistical Software},
    year = {2009},
    volume = {32},
    number = {8},
    pages = {1--27},
    url = {http://www.jstatsoft.org/v32/i08/},
  }