Moses Charikar Sudipto Guha Éva Tardos David B. Shmoys A Constant-Factor Approximation Algorithm for the k-Median Problem. 129-149 2002 65 J. Comput. Syst. Sci. 1 http://dx.doi.org/10.1006/jcss.2002.1882 db/journals/jcss/jcss65.html#CharikarGTS02