Giorgos Giannopoulos Theodore Dalamagas Magdalini Eirinaki Timos K. Sellis Boosting the ranking function learning process using clustering. 125-132 2008 WIDM http://doi.acm.org/10.1145/1458502.1458523 conf/widm/2008 db/conf/widm/widm2008.html#GiannopoulosDES08