To cite the R package 'cluster' in publications use:

Maechler M, Rousseeuw P, Struyf A, Hubert M, Hornik K (2021). cluster: Cluster Analysis Basics and Extensions. R package version 2.1.2 — For new features, see the 'Changelog' file (in the package source), https://CRAN.R-project.org/package=cluster.

Corresponding BibTeX entry:

  @Manual{,
    title = {cluster: Cluster Analysis Basics and Extensions},
    author = {Martin Maechler and Peter Rousseeuw and Anja Struyf and
      Mia Hubert and Kurt Hornik},
    year = {2021},
    url = {https://CRAN.R-project.org/package=cluster},
    note = {R package version 2.1.2 --- For new features, see the
      'Changelog' file (in the package source)},
  }