To cite extremefit in publications use:

Durrieu G, Grama I, Jaunatre K, Pham Q, Tricot J (2018). “extremefit: A Package for Extreme Quantiles.” Journal of Statistical Software, 87(12), 1–20. doi:10.18637/jss.v087.i12.

Corresponding BibTeX entry:

    title = {{extremefit}: A Package for Extreme Quantiles},
    author = {Gilles Durrieu and Ion Grama and Kevin Jaunatre and
      Quang-Khoai Pham and Jean-Marie Tricot},
    journal = {Journal of Statistical Software},
    year = {2018},
    volume = {87},
    number = {12},
    pages = {1--20},
    doi = {10.18637/jss.v087.i12},