To cite frailtypack in publications use:

Virginie Rondeau, Yassin Mazroui, Juan R. Gonzalez (2012). frailtypack: An R Package for the Analysis of Correlated Survival Data with Frailty Models Using Penalized Likelihood Estimation or Parametrical Estimation. Journal of Statistical Software, 47(4), 1-28. URL http://www.jstatsoft.org/v47/i04/.
Virginie Rondeau, Juan R. Gonzalez (2005). {Frailtypack}: A computer program for the analysis of correlated failure time data using penalized likelihood estimation Computer Methods and Programs in Biomedicine, 80(2), 154-164.

Corresponding BibTeX entries:

  @Article{,
    title = {{frailtypack}: An {{R}} Package for the Analysis of
      Correlated Survival Data with Frailty Models Using Penalized
      Likelihood Estimation or Parametrical Estimation},
    author = {Virginie Rondeau and Yassin Mazroui and Juan R.
      Gonzalez},
    journal = {Journal of Statistical Software},
    year = {2012},
    volume = {47},
    number = {4},
    pages = {1-28},
    url = {http://www.jstatsoft.org/v47/i04/},
  }
  @Article{,
    title = {{Frailtypack}: A computer program for the analysis of
      correlated failure time data using penalized likelihood
      estimation},
    author = {Virginie Rondeau and Juan R. Gonzalez},
    journal = {Computer Methods and Programs in Biomedicine},
    year = {2005},
    volume = {80},
    number = {2},
    pages = {154-164},
  }