Alon Shalita, Uri Zwick. Efficient algorithms for the 2-gathering problem. In Claire Mathieu, editor, Proceedings of the Twentieth Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2009, New York, NY, USA, January 4-6, 2009. pages 96-105, SIAM, 2009. [doi]
Abstract is missing.