S. Kaslusky, D. Sabatino, and L. Zeidner. Proceedings of the 45th AIAA Aerospace Sciences Meeting and Exhibit, American Institute of Aeronautics and Astronautics, (January 2007)
R. Mehrotra, S. Sanner, W. Buntine, and L. Xie. Proceedings of the 36th international ACM SIGIR conference on Research and development in information retrieval, page 889--892. ACM, (2013)
S. Tremaine. (2014)cite arxiv:1407.6363Comment: 14 pages, 2 figures. To appear in "From Atoms to the Stars", a special issue of Daedalus (Fall 2014, vol. 143, no. 4).