To cite frailtyEM in publications use:

  Balan TA, Putter H (2019). “frailtyEM: An R Package for Estimating
  Semiparametric Shared Frailty Models.” _Journal of Statistical
  Software_, *90*(7), 1-29. doi:10.18637/jss.v090.i07
  <https://doi.org/10.18637/jss.v090.i07>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{frailtyEM}: An {R} Package for Estimating Semiparametric Shared Frailty Models},
    author = {Theodor Adrian Balan and Hein Putter},
    journal = {Journal of Statistical Software},
    year = {2019},
    volume = {90},
    number = {7},
    pages = {1--29},
    doi = {10.18637/jss.v090.i07},
  }