Jiaqian Zheng Xiaoyuan Wu Junyu Niu Alvaro Bolivar Substitutes or complements: another step forward in recommendations. 139-146 2009 ACM Conference on Electronic Commerce http://doi.acm.org/10.1145/1566374.1566394 conf/sigecom/2009 db/conf/sigecom/sigecom2009.html#ZhengWNB09