Yunjun Gao Baihua Zheng Gencai Chen Qing Li On efficient mutual nearest neighbor query processing in spatial databases. 705-727 2009 68 Data Knowl. Eng. 8 http://dx.doi.org/10.1016/j.datak.2009.04.004 db/journals/dke/dke68.html#GaoZCL09