J. Yoon, B. Noble, M. Liu, and M. Kim. International Conference on Mobile Systems, Applications and Services, page 177--190. New York, NY, USA, ACM, (2006)
C. Trattner, D. Helic, P. Singer, and M. Strohmaier. Proceedings of the 12th International Conference on Knowledge Management and Knowledge Technologies, page 14. ACM, (2012)