Süleyman Cenk Sahinalp Andrey Utis Hardness of String Similarity Search and Other Indexing Problems. 1080-1098 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3142&spage=1080 2004 conf/icalp/2004 ICALP db/conf/icalp/icalp2004.html#SahinalpU04