Complexity of Manipulation in Elections with Top-truncated Ballots (1505.05900v2)

Abstract: In the computational social choice literature, there has been great interest in understanding how computational complexity can act as a barrier against manipulation of elections. Much of this literature, however, makes the assumption that the voters or agents specify a complete preference ordering over the set of candidates. There are many multiagent systems applications, and even real-world elections, where this assumption is not warranted, and this in turn raises the question "How hard is it to manipulate elections if the agents reveal only partial preference orderings?" It is this question we try to address in this paper. In particular, we look at the weighted manipulation problem -- both constructive and destructive manipulation -- when the voters are allowed to specify any top-truncated ordering over the set of candidates. We provide general results for all scoring rules, for elimination versions of all scoring rules, for the plurality with runoff rule, for a family of election systems known as Copeland$^{\alpha}$, and for the maximin protocol. Finally, we also look at the impact on complexity of manipulation when there is uncertainty about the non-manipulators' votes.