Venkatesan Guruswami Rajsekar Manokaran Prasad Raghavendra Beating the Random Ordering is Hard: Inapproximability of Maximum Acyclic Subgraph. 573-582 2008 FOCS http://dx.doi.org/10.1109/FOCS.2008.51 conf/focs/2008 db/conf/focs/focs2008.html#GuruswamiMR08