The Complexity of Manipulating $k$-Approval Elections

TL;DR

This paper investigates the computational complexity of manipulation in $k$-approval and related election systems, revealing a classification of these problems and uncovering connections to graph theory.

Contribution

It generalizes the complexity analysis of election manipulation to infinitely many scoring protocols and unbounded candidates, providing a comprehensive classification.

Findings

Problems are classified as polynomial-time solvable, NP-hard, or equivalent to other problems.

Established a connection between election manipulation and graph theory problems.

Extended previous fixed-candidate results to unbounded candidate scenarios.

Abstract

An important problem in computational social choice theory is the complexity of undesirable behavior among agents, such as control, manipulation, and bribery in election systems. These kinds of voting strategies are often tempting at the individual level but disastrous for the agents as a whole. Creating election systems where the determination of such strategies is difficult is thus an important goal. An interesting set of elections is that of scoring protocols. Previous work in this area has demonstrated the complexity of misuse in cases involving a fixed number of candidates, and of specific election systems on unbounded number of candidates such as Borda. In contrast, we take the first step in generalizing the results of computational complexity of election misuse to cases of infinitely many scoring protocols on an unbounded number of candidates. Interesting families of systems include $k$-approval and $k$-veto elections, in which voters distinguish $k$ candidates from the candidate set. Our main result is to partition the problems of these families based on their complexity. We do so by showing they are polynomial-time computable, NP-hard, or polynomial-time equivalent to another problem of interest. We also demonstrate a surprising connection between manipulation in election systems and some graph theory problems.