Maria Halkidi Michalis Vazirgiannis Clustering Validity Assessment: Finding the Optimal Partitioning of a Data Set. 187-194 2001 conf/icdm/2001 ICDM http://doi.ieeecomputersociety.org/10.1109/ICDM.2001.989517 db/conf/icdm/icdm2001.html#HalkidiV01