L. von Ahn, и L. Dabbish. CHI '04: Proceedings of the SIGCHI conference on Human factors in computing systems, стр. 319--326. New York, NY, USA, ACM Press, (2004)
J. Ke, T. Gong, и W. Wang. Int. J. Mod. Phys. C, (2007)Originally presented at the The 5th Conference on Language evolution, Leipzig, Germany, March 2004..