Control by Deleting Players from Weighted Voting Games Is NP^PP-Complete for the Penrose-Banzhaf Power Index

Abstract

Weighted voting games are a popular class of coalitional games that are widely used to model real-life situations of decision-making. They can be applied, for instance, to analyze legislative processes in parliaments or voting in corporate structures. Various ways of tampering with these games have been studied, among them merging or splitting players, fiddling with the quota, and controlling weighted voting games by adding or deleting players. While the complexity of control by adding players to such games so as to change or maintain a given player's power has been recently settled, the complexity of control by deleting players from such games (with the same goals) remained open. We show that when the players' power is measured by the probabilistic Penrose-Banzhaf index, some of these problems are complete for NP^PP -- the class of problems solvable by NP machines equipped with a PP ("probabilistic polynomial time") oracle. Our results optimally improve the currently known lower bounds of hardness for much smaller complexity classes, thus providing protection against SAT-solving techniques in practical applications.