Feodor F. Dragan Chenyu Yan Irina Lomonosov Collective Tree Spanners of Graphs. 64-76 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3111&spage=64 2004 conf/swat/2004 SWAT db/conf/swat/swat2004.html#DraganYL04