A tournament is an orientation of a complete graph. A feedback arc set is a set of arcs in a digraph whose removal leave the digraph acyclic. The feedback arc set problem consists in finding a feedback arc set of minimum size. A polynomial time approximation scheme is an algorithm which takes an instance of an optimization problem and a parameter $ \epsilon > 0 $ and, in polynomial time, produces a solution that is within a factor $ 1+\epsilon $ of being optimal.
The feedback arc set problem has been proved NP-hard. See [ACM, A, CTY, C]. It was shown in [RS] that the feedback arc set problem is fixed parameter tractable for tournaments.
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[A] N. Alon, Ranking tournaments, SIAM J. Discrete Math. 20 (2006) 137–142.
[CTY] P. Charbit, P. Thomassé, A. Yeo, The minimum Feedback arc set problem is NP-hard for tournaments, Combin. Probab. Comput. 16 (1) (2007) 1–4.
[C] V. Conitzer, Computing Slater rankings using similarities among candidates, in: Proceedings, The Twenty-First National Conference on Artificial Intelligence and the Eighteenth Innovative Applications of Artificial Intelligence Conference, July 16–20, AAAI Press, Boston, Massachusetts, USA, 2006.
[RS] V. Raman, S. Saurabh, Parameterized complexity of directed feedback arc set problems in tournaments, in: Algorithms and Data Structures, in: Lecture Notes in Computer Science, vol. 2748, Springer, Berlin, 2003, pp. 484–492.