Unweighted Coalitional Manipulation under the Borda Rule Is NP-Hard

@inproceedings{Betzler2011UnweightedCM,
  title={Unweighted Coalitional Manipulation under the Borda Rule Is NP-Hard},
  author={Nadja Betzler and Rolf Niedermeier and Gerhard J. Woeginger},
  booktitle={IJCAI},
  year={2011}
}
The Borda voting rule is a positional scoring rule where, for m candidates, for every vote the first candidate receives m- 1 points, the second m- 2 points and so on. A Borda winner is a candidate with highest total score. It has been a prominent open problem to determine the computational complexity of UNWEIGHTED COALITIONAL MANIPULATION UNDER BORDA: Can one add a certain number of additional votes (called manipulators) to an election such that a distinguished candidate becomes a winner? We… 

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