Piotr Berman Bhaskar DasGupta Ming-Yang Kao Tight approximability results for test set problems in bioinformatics. 145-162 2005 71 J. Comput. Syst. Sci. 2 http://dx.doi.org/10.1016/j.jcss.2005.02.001 db/journals/jcss/jcss71.html#BermanDK05