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Prove or disprove the following statements by either giving a formal proof or a counter-example.(a) Given any number of agents and their complete preference profile over a set of outcomesO, if |O| = 2 then Plurality voting will always select the Condorcet winner.(b) Borda voting will always select the Condorcet winner if the possible number of outcomesis exactly 3, ie. |O| = 3, and n > 999 where n is the number of agents.

Question

Prove or disprove the following statements by either giving a formal proof or a counter-example.(a) Given any number of agents and their complete preference profile over a set of outcomesO, if |O| = 2 then Plurality voting will always select the Condorcet winner.(b) Borda voting will always select the Condorcet winner if the possible number of outcomesis exactly 3, ie. |O| = 3, and n > 999 where n is the number of agents.

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Solution

(a) This statement is true. The Condorcet winner is the candidate who would win a head-to-head competition against each of the other candidates. When there are only two outcomes, the Condorcet winner is simply the candidate preferred by the majority of agents. Plurality voting, which selects the candidate with the most votes, will also select this candidate. Therefore, when |O| = 2, Plurality voting will always select the Condorcet winner.

(b) This statement is false. Borda voting does not always select the Condorcet winner, even when the number of outcomes is exactly 3 and the number of agents is greater than 999.

Here is a counter-example:

Suppose there are 1000 agents and 3 outcomes A, B, and C.

  • 501 agents rank the outcomes as A > B > C
  • 499 agents rank the outcomes as B > C > A

In a head-to-head competition, B would win against both A and C, so B is the Condorcet winner.

However, in Borda voting, each agent assigns points to the outcomes based on their rankings (2 points for their top choice, 1 point for their second choice, and 0 points for their last choice).

  • A receives 5012 + 4990 = 1002 points
  • B receives 5011 + 4992 = 1498 points
  • C receives 5010 + 4991 = 499 points

So, A would win in Borda voting, even though B is the Condorcet winner. Therefore, Borda voting does not always select the Condorcet winner.

This problem has been solved

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