1 Introduction
Information overload is an increasing problem in people's every life due to the proliferation of the Internet. Recommender system serves as an effective solution to alleviate the information overload issue, to facilitate users seeking desired information, and to increase the traffic and revenue of service providers. It has been used in a wide range of applications, such as e-commerce, social media sites, news portals, app stores, digital libraries, and so on. It is one of the most ubiquitous user-centered artificial intelligence applications in modern information systems.