To cite perm in publications use:

Fay MP, Shaw PA (2010). “Exact and Asymptotic Weighted Logrank Tests for Interval Censored Data: The interval R Package.” Journal of Statistical Software, 36(2), 1–34. doi:10.18637/jss.v036.i02.

Corresponding BibTeX entry:

  @Article{,
    title = {Exact and Asymptotic Weighted Logrank Tests for Interval
      Censored Data: The {interval} {R} Package},
    author = {Michael P. Fay and Pamela A. Shaw},
    journal = {Journal of Statistical Software},
    year = {2010},
    volume = {36},
    number = {2},
    pages = {1--34},
    doi = {10.18637/jss.v036.i02},
  }