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A Simple Proof "There is no consistent method by which a democratic society can make a choice (when voting) that is always fair when that choice must be made from among three or more alternatives."

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Let A be a set of outcomes, N a number of voters or decision criteria. The set of all full linear orderings of A is then denoted by by L(A). Note: This set is equivalent to the set S | A | of permutations on the elements of A). A social welfare function is a function, F: L(A) N →L(A), which aggregates voters' preferences into a single preference order on A. The N-tuple (R 1, …, R N ) of voter's preferences is called a preference profile.

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Arrow's Impossibility theorem states that whenever the set A of possible alternatives has more than 2 elements, then the following three conditions, called fairness criteria become incompatible: Unanimity (Pareto efficiency): If alternative a is ranked above b for all orderings R 1, …, R N, then a is ranked higher than b by F(R 1, …, R N ). (Note that unanimity implies non- imposition). Non-dictatorship: There is no individual i whose preferences always prevail. That is, there is no i є {1, …,N} such that for every (R 1, …, R N ) є L(A) N, F(R 1, …, R N ) = R i. Independence of Irrelevant Alternatives: For two preference profiles (R 1, …, R N ) and (S 1, …, S N ) such that for all individuals i, alternatives a and b have the same order in R i as in S i, alternatives a and b have the same order in F(R 1, …, R N ) as in F(S 1, …, S N ). Independence of Irrelevant Alternatives http://en.wikipedia.org/wiki/Arrow%27s_impossibility_theorem Fairness CriteriaFairness Criteria

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Commonly restated as: "No voting method is fair", "Every ranked voting method is flawed", or "The only voting method that isn't flawed is a dictatorship". But these are oversimplified, and thus do not hold universally Actually says: A voting mechanism can’t follow all the fairness criteria for all possible preference orders Any social choice system respecting unrestricted domain, unanimity, and independence of irrelevant alternatives is a dictatorship http://en.wikipedia.org/wiki/Arrow%27s_impossibility_theorem

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Has been proven in numerous ways Graph Theory Proof, from a paper by Nambiar, Varma and Saroch, submitted May 1992. www.ece.rutgers.edu/~knambiar/science/ArrowProof.pdf

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Uses two digraphs, D=(V,A), a preference and anonpreference. Nonpreference: complete and transitive Preference graph is the complement of the nonpreference graph Adjacency matrix of preference graph is called the preference matrix

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Notation: m is the total number of candidates C 1, …….., C m n is the total number of voters V 1, …….., V n V k = [v i, j k ] is the preference matrix of order m by m which gives the preference of the voter, V k, where k is an element of {1, …, n}. When v i, j k = 0, the voter does not prefer candidate i over candidate j. When v i,j k = 1, the voter prefers candidate i over candidate j. 0 (boldface) represents a nonpreference set of voters, while 1 (also boldface) represents a preference set of voters. v i, j k = * means that the voter has an unspecified preference. The star also represents the unspecified preference set of voters.

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S = [s i j ] is the preference matrix of m by m order which gives the preference of society as a whole, rather than individual voters. The voting function is F(V 1, …….., V n ) = S. The dictator function is also a projection function and is as follows: D n k (x 1, ….x n )= x k.

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(Previously mentioned under Fairness criteria) Axiom of Independence S ij = f ij (v ij 1,……..v ij n ) for i≠j and s ij =0 States that s ij is a function of the v ij k ‘s only Axiom of Unanimity f ij (0,0,…..0)=f ij (1,1,…..1)=1 States that if all the voters vote one way then the voting system also votes the same way (definition of unanimous)

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We want to prove: f ij (x 1, …..x n )= D n d (x 1, ….x n )= x d which means that S= V d

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Proof: Define: h= min ij {sum from k=1,…..n of the x k so that f ij (x 1,...,x n )=1} Note that the m(m-1)2 n values of f ij are to be inspected before we can obtain the value of h. We want to show that h = 1. f ij( 1; 0; 0) = 0 and f jk (1; 0; 0) = 0 →f ik (*,*,0) = 0 since nonpreference graphs are transitive → f ik (1; 1; 0) = 0 Taking the contrapositive of the above argument f ik (1; 1; 0) = 1 → f ij (1; 0; 0) = 1 or f jk (0; 1; 0) = 1

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It immediately follows that h = 1. Note that h cannot be zero because of the Unanimity axiom. Without loss of generality we may assume f ab (1; 0) = 1, the position at which 1 occurs in f ab is of no concern to us. Here C a and C b are two specific candidates. Now, f ia (1; 1) = 1 and f ab (1; 0) = 1 →f ib (1; *) = 1 since preference graphs are transitive, and f ib (1; *) = 1 and f bj (1; 1) = 1 → f ij (1;*) = 1 since preference graphs are transitive → f ji (0; *) = 0 since preference graphs are asymmetric → f ij (x 1 ; *) = x 1 Dictator Theorem immediately follows.

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Can be extended to symmetric tournaments Has been proven in several other (more complicated) ways using graph theory

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Beigman, Eyal. “Extension of Arrow’s Theorem to Symmetric Sets of Tournaments.” Discrete Mathematics. Vol 301 pg 2074-2081. Namblar, K.K., Prarnod K. Varma and Vandana Saroch. “A Graph Theoretic Proof of Arrow’s Dictator Theorem.” May 1992. Powers, R.C. “Arrow’s Theorem for Closed Weak Hierarchies.” Discrete Applied Mathematics. Vol 66 pg 271-278.

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Social choice theory = preference aggregation = voting assuming agents tell the truth about their preferences Tuomas Sandholm Professor Computer Science.

Social choice theory = preference aggregation = voting assuming agents tell the truth about their preferences Tuomas Sandholm Professor Computer Science.

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