Hiroshi Nagamochi Kohei Okada Polynomial Time 2-Approximation Algorithms for the Minmax Subtree Cover Problem. 138-147 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2906&spage=138 2003 conf/isaac/2003 ISAAC db/conf/isaac/isaac2003.html#NagamochiO03