A Complexity Theoretical Study of Fuzzy K-Means
J. Blömer, S. Brauer, K. Bujna, ACM Transactions on Algorithms 16 (2020) 1–25.

Clustering for Metric and Nonmetric Distance Measures
M.R. Ackermann, J. Blömer, C. Sohler, ACM Trans. Algorithms (2010) 59:1--59:26.