http://machinelearningmastery.com/an-introduction-to-feature-selection/ http://machinelearningmastery.com/feature-selection-in-python-with-scikit-learn/ http://stats.stackexchange.com/questions/40576/is-using-the-same-data-for-feature-selection-and-cross-validation-biased-or-not
http://www.slideshare.net/irecsys/matrix-factorization-in-recommender-systems
http://derekgreene.com/slides/nmf_insight_workshop.pdf
http://dufu.math.ncu.edu.tw/calculus/calculus_eng/node162.html