Clustering: http://www.cs.ccsu.edu/~markov/ccsu_courses/DataMining-10.pdf

Density-based clustering: http://www.cse.buffalo.edu/faculty/azhang/cse601/density-based.ppt

Conceptual Clustering: http://users.ics.forth.gr/~potamias/hy577/cc.pdf

Ester, Martin; Kriegel, Hans-Peter; Sander, Jörg; Xu, Xiaowei (1996). Simoudis, Evangelos; Han, Jiawei; Fayyad, Usama M., eds. A density-based algorithm for discovering clusters in large spatial databases with noise. Proceedings of the Second International Conference on Knowledge Discovery and Data Mining (KDD-96). AAAI Press. pp. 226–231.

Michalski R.S., Stepp R.E. (1983) Learning from Observation: Conceptual Clustering. In: Michalski R.S., Carbonell J.G., Mitchell T.M. (eds) Machine Learning. Symbolic Computation. Springer, Berlin, Heidelberg, pp. 331-363.

Douglas H. Fisher, Knowledge Acquisition Via Incremental Conceptual Clustering. In: Jude W. Shavlik, Thomas G. Dietterich (Eds.), Readings in Machine Learning, 1987, pp. 267-283.