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Prof. Dr. Jürgen Dix · Department of Informatics, TUC <strong>Multiagent</strong> <strong>Systems</strong>, WS 06/07 285/731<br />

4. Decision Making (2) 1. Voting<br />

Proof (of Arrows theorem).<br />

The proof is the third proof given by John<br />

Geanakoplos (1996) and based on the following<br />

Lemma 4.12 (Strict Neutrality)<br />

Consider two pairs of alternatives a, b and α, β.<br />

Suppose each voter strictly prefers a to b or b to<br />

a, i.e. for all i: a≺ ≺ i b or b≺ ≺ i a<br />

Suppose further that each voter has the same<br />

preference for α, β as she has for a, b.<br />

Then either a≺ ≺ ∗ b and α ≺ ∗ β or b≺ ≺ ∗ a and β ≺ ∗ α.

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