B. Sarwar, G. Karypis, J. Konstan, and J. Reidl. Proceedings of the 10th International Conference on World Wide Web, page 285--295. New York, NY, USA, ACM, (2001)
R. Krestel, P. Fankhauser, and W. Nejdl. RecSys '09: Proceedings of the third ACM conference on Recommender systems, page 61--68. New York, NY, USA, ACM, (2009)