Vladimir Estivill-Castro Michael E. Houle Data Structures for Minimization of Total Within-Group Distance for Spatio-temporal Clustering. 91-102 2001 conf/pkdd/2001 PKDD http://dx.doi.org/10.1007/3-540-44794-6_8 db/conf/pkdd/pkdd2001.html#Estivill-CastroH01