“…Clustering a set of points into a few groups is frequently used for statistical analysis and classi cation in numerous applications, including information retrieval 6,7,8,26], facility location 11,30], data mining 2,28], spatial data bases 9,20,25], data compression 23], image processing 19, 27], astrophysics 22], and scienti c computing 5]. Because of such a diversity of applications, several variants of clustering problems have been proposed and widely studied.…”