To cite fastcluster in publications use:
Müllner D (2013). “fastcluster: Fast Hierarchical, Agglomerative Clustering Routines for R and Python.” Journal of Statistical Software, 53(9), 1–18. https://www.jstatsoft.org/v53/i09/.
Corresponding BibTeX entry:
@Article{, title = {{fastcluster}: Fast Hierarchical, Agglomerative Clustering Routines for {R} and {Python}}, author = {Daniel M\"ullner}, journal = {Journal of Statistical Software}, year = {2013}, volume = {53}, number = {9}, pages = {1--18}, url = {https://www.jstatsoft.org/v53/i09/}, }