[1]PMF:Probabilistic Matrix Factorization: http://papers.nips.cc/paper/3208-probabilistic-matrix-factorization.pdf
[2]socialMF:A Matrix Factorization Technique with Trust Propagation for Recommendation in Social Networks http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.459.691&rep=rep1&type=pdf
[3]RSTE:Learning to Recommend with Social Trust Ensemble:http://www.cc.gatech.edu/~zha/CSE8801/CF/p203-ma.pdf
[4]CBR:Circle-based Recommendation in Online Social Networks http://eeweb.poly.edu/faculty/yongliu/docs/CircleRec.pdf