To cite survPen in publications use:

  Mathieu Fauvernier, Laurent Remontet, Zoé Uhry, Nadine Bossard and
  Laurent Roche (2019). survPen: an R package for hazard and excess
  hazard modelling with multidimensional penalized splines. Journal of
  Open Source Software, 4(40), 1434. doi: 10.21105/joss.01434

A BibTeX entry for LaTeX users is

  @Article{,
    title = {survPen: an R package for hazard and excess hazard modelling with multidimensional penalized splines},
    author = {Mathieu Fauvernier and Laurent Remontet and Zoé Uhry and Nadine Bossard and Laurent Roche},
    journal = {Journal of Open Source Software},
    year = {2019},
    volume = {4},
    number = {40},
    pages = {1434},
    doi = {10.21105/joss.01434},
  }