Guy Even Naveen Garg Jochen Könemann R. Ravi Amitabh Sinha Covering Graphs Using Trees and Stars. 24-35 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2764&spage=24 2003 conf/random/2003 RANDOM-APPROX db/conf/random/random2003.html#EvenGKRS03