The Banzhaf form of Penrose's limit theorem

Let WR0W\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=0N(n)N=\bigcup_{n=0}^{\infty}N^{(n)}. Assign each voter iNi\in N a weight wiWw_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 qiN(n)wiq\cdot\sum_{i\in N^{(n)}}w_i. A voter ii is regular if there are n0Nn_0\in\mathbb{N} and ε>0\varepsilon>0 such that

{hN(n)wh=wi}wihN(n)whε\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 nn0n\ge n_0, and the chain is non-atomic if limnwi/hN(n)wh=0\lim_{n\to\infty}w_i/\sum_{h\in N^{(n)}}w_h=0 for every iNi\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,jNi,j\in N with nonzero weights,

limnBzi(W(n))Bzj(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.

Sources & referencesView supporting material

Primary source

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

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