To cite the 'rtk' package in publications use:

Saary P, Forslund K, Bork P and Hildebrand F (2017). RTK: efficient rarefaction analysis of large datasets. doi: 10.1093/bioinformatics/btx206, R package version 0.2.5.4, https://github.com/hildebra/Rarefaction.

Corresponding BibTeX entry:

  @Manual{,
    title = {RTK: efficient rarefaction analysis of large datasets},
    author = {Paul Saary and Kristoffer Forslund and Peer Bork and Falk
      Hildebrand},
    year = {2017},
    note = {R package version 0.2.5.4},
    doi = {10.1093/bioinformatics/btx206},
    journal = {Bioinformatics},
    url = {https://github.com/hildebra/Rarefaction},
  }