J. Tang, J. Zhang, L. Yao, J. Li, L. Zhang, and Z. Su. Proceedings of the 14th ACM SIGKDD International Conference on Knowledge Discovery and Data Mining, page 990--998. New York, NY, USA, ACM, (2008)
R. Jäschke, A. Hotho, F. Mitzlaff, and G. Stumme. Recommender Systems for the Social Web, volume 32 of Intelligent Systems Reference Library, Springer, Berlin/Heidelberg, (2012)