To cite package 'fastcluster' in publications use:

Daniel Müllner, fastcluster: Fast hierarchical clustering routines for R and Python, Version 1.1.6, 2012. Available at CRAN http://cran.r-project.org and http://math.stanford.edu/~muellner.

Corresponding BibTeX entry:

  @Manual{,
    title = {fastcluster: Fast hierarchical clustering routines for R
      and Python},
    author = {Daniel M\"ullner},
    year = {2012},
    note = {Version 1.1.6},
    url = {http://math.stanford.edu/~muellner},
  }