Lops2011Content-based.md

@article{Lops2011Content-based,
  title={Content-based recommender systems: State of the art and trends},
  author={Lops, Pasquale and De Gemmis, Marco and Semeraro, Giovanni},
  journal={Recommender systems handbook},
  pages={73--105},
  year={2011},
  publisher={Springer}
}