The Complexity of Power-Index Comparison

Piotr Faliszewski; Lane A. Hemaspaandra

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arXiv · 0801.4585

The Complexity of Power-Index Comparison

Piotr Faliszewski·Lane A. Hemaspaandra

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Abstract

We study the complexity of the following problem: Given two weighted voting games G' and G'' that each contain a player p, in which of these games is p's power index value higher? We study this problem with respect to both the Shapley-Shubik power index [SS54] and the Banzhaf power index [Ban65,DS79]. Our main result is that for both of these power indices the problem is complete for probabilistic polynomial time (i.e., is PP-complete). We apply our results to partially resolve some recently proposed problems regarding the complexity of weighted voting games. We also study the complexity of the raw Shapley-Shubik power index. Deng and Papadimitriou [DP94] showed that the raw Shapley-Shubik power index is #P-metric-complete. We strengthen this by showing that the raw Shapley-Shubik power index is many-one complete for #P. And our strengthening cannot possibly be further improved to parsimonious completeness, since we observe that, in contrast with the raw Banzhaf power index, the raw Shapley-Shubik power index is not #P-parsimonious-complete.

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Piotr Faliszewski, Lane A. Hemaspaandra. 2008-01-30. The Complexity of Power-Index Comparison. https://arxiv.org/abs/0801.4585

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