I. Introduction
Recommender system is one of the most successful data mining technologies nowadays. It was introduced in late 1980’s and 1990’s and quickly developed into a major research field in artificial intelligence area. Unlike other artificial intelligence technologies such as natural language processing and computer vision, recommender system became commercializable fairly quickly. Important application scenarios of recommender systems include E-commerce websites, cultural record websites, music streaming services, social networks, and video social networks.