The Banzhaf form of Penrose's limit theorem

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Let W⊆R≥0W\subseteq\mathbb{R}_{\ge 0} be a set of weights, let N(0)⊊N(1)⊊…N^{(0)}\subsetneq N^{(1)}\subsetneq\dots be an infinite increasing chain of finite non-empty sets, and put N=⋃n=0∞N(n)N=\bigcup_{n=0}^{\infty}N^{(n)}. Assign each voter i∈Ni\in N a weight wi∈Ww_i\in W. For a fixed quota q∈(0,1)q\in(0,1), let W(n)\mathcal{W}^{(n)} be the weighted majority game with voter set N(n)N^{(n)}, weights wiw_i and quota q⋅∑i∈N(n)wiq\cdot\sum_{i\in N^{(n)}}w_i. A voter ii is regular if there are n0∈Nn_0\in\mathbb{N} and ε>0\varepsilon>0 such that

∣{h∈N(n)∣wh=wi}∣⋅wi∑h∈N(n)wh≥ε\left|\left\{h\in N^{(n)}\mid w_h=w_i\right\}\right|\cdot\frac{w_i}{\sum_{h\in N^{(n)}}w_h}\ge\varepsilon

for all n≥n0n\ge n_0, and the chain is non-atomic if lim⁡n→∞wi/∑h∈N(n)wh=0\lim_{n\to\infty}w_i/\sum_{h\in N^{(n)}}w_h=0 for every i∈Ni\in N. Banzhaf–Penrose limit conjecture. Penrose's limit theorem holds for regular players in non-atomic chains for the Banzhaf index with every relative quota q∈(0,1)q\in(0,1); that is, for every pair i,j∈Ni,j\in N with nonzero weights,

lim⁡n→∞Bz⁡i(W(n))Bz⁡j(W(n))=wiwj.\lim_{n\to\infty}\frac{\operatorname{Bz}_i(\mathcal{W}^{(n)})}{\operatorname{Bz}_j(\mathcal{W}^{(n)})}=\frac{w_i}{w_j}.

The corresponding result is known for the Banzhaf index at relative quota q=12q=\frac12, while the arbitrary-quota case remains open.

References

Primary source

Sascha Kurz, “Ready for the design of voting rules?”, arXiv:1405.0823 (2014).

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