Control by Adding Players to Change or Maintain the Shapley-Shubik or the Penrose-Banzhaf Power Index in Weighted Voting Games Is Complete for NP^PP

TL;DR

This paper proves that controlling the addition of players to weighted voting games to alter or preserve power indices is computationally complete for the class NP^PP, highlighting the problem's inherent complexity.

Contribution

It establishes NP^PP-completeness for control problems in weighted voting games, improving the known lower bounds and providing a new proof technique.

Findings

NP^PP-hardness of control problems for power indices

Completeness results for these problems in NP^PP

Potential applicability of proof techniques to other problems

Abstract

Weighted voting games are a well-known and useful class of succinctly representable simple games that have many real-world applications, e.g., to model collective decision-making in legislative bodies or shareholder voting. Among the structural control types being analyzing, one is control by adding players to weighted voting games, so as to either change or to maintain a player's power in the sense of the (probabilistic) Penrose-Banzhaf power index or the Shapley-Shubik power index. For the problems related to this control, the best known lower bound is PP-hardness, where PP is "probabilistic polynomial time," and the best known upper bound is the class NP^PP, i.e., the class NP with a PP oracle. We optimally raise this lower bound by showing NP^PP-hardness of all these problems for the Penrose-Banzhaf and the Shapley-Shubik indices, thus establishing completeness for them in that class. Our proof technique may turn out to be useful for solving other open problems related to weighted voting games with such a complexity gap as well.