To cite the R package cluster in publications use:

  Maechler, M., Rousseeuw, P., Struyf, A., Hubert, M., Hornik,
  K.(2023).  cluster: Cluster Analysis Basics and Extensions. R package
  version 2.1.6.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {cluster: Cluster Analysis Basics and Extensions},
    author = {Martin Maechler and Peter Rousseeuw and Anja Struyf and Mia Hubert and Kurt Hornik},
    year = {2023},
    url = {https://CRAN.R-project.org/package=cluster},
    note = {R package version 2.1.6 --- For new features, see the 'NEWS' and the 'Changelog' file in the package source)},
  }