Marti A. Hearst Jan O. Pedersen Reexamining the Cluster Hypothesis: Scatter/Gather on Retrieval Results. 76-84 1996 SIGIR db/conf/sigir/sigir96.html#HearstP96 conf/sigir/96 SIGIR1996/P076.pdf http://doi.acm.org/10.1145/243199.243216