Graham Cormode Srikanta Tirthapura Bojian Xu Time-Decayed Correlated Aggregates over Data Streams. 269-280 2009 SDM http://www.siam.org/proceedings/datamining/2009/dm09_027_cormodeg.pdf conf/sdm/2009 db/conf/sdm/sdm2009.html#CormodeTX09