Michael Elkin Guy Kortsarz Polylogarithmic Inapproximability of the Radio Broadcast Problem. 105-116 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3122&spage=105 2004 conf/approx/2004 APPROX-RANDOM db/conf/approx/approx2004.html#ElkinK04