Lars Engebretsen Jonas Holmerin Alexander Russell Inapproximability Results for Equations over Finite Groups. 73-84 2002 conf/icalp/2002 ICALP http://link.springer.de/link/service/series/0558/bibs/2380/23800073.htm db/conf/icalp/icalp2002.html#EngebretsenHR02