How Does Ivanhoe Stay Ahead in the Pirate Captain Vote?

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Discussion Overview

The discussion revolves around a voting scenario involving two pirates, Ivanhoe and Isaac, who are competing for the position of captain. Participants explore the probability of Ivanhoe leading throughout the voting process, given that he receives more votes than Isaac. The conversation touches on the mathematical underpinnings of this scenario, referencing known problems in probability theory.

Discussion Character

  • Exploratory, Technical explanation, Mathematical reasoning

Main Points Raised

  • One participant poses a question about the probability of Ivanhoe leading during the voting process, given that he receives more votes than Isaac.
  • Another participant provides partial results that are acknowledged as correct and close to a general solution.
  • A different participant mentions discovering a simpler explanation for the problem, identifying it as the "Ballot problem" and noting the existence of geometrical proofs and generalized versions.
  • The same participant shares a link to a resource that elaborates on the Ballot problem, suggesting that it contains comprehensive information on the topic.

Areas of Agreement / Disagreement

Participants appear to agree on the correctness of the partial results, but there is no consensus on the simplest approach to the problem or the best method for understanding it.

Contextual Notes

The discussion references the "Ballot problem," which may imply certain mathematical assumptions and definitions that are not fully explored within the thread.

Who May Find This Useful

Readers interested in probability theory, mathematical problems related to voting scenarios, or those looking for insights into the Ballot problem may find this discussion relevant.

humanino
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Ivanhoe and Isaac became pirates a long time ago. Both of them run for the position of captain. Everybody slips his envelop in the box one after the other. Ivanhoe receives P votes, and Isaac receives Q votes. Assuming P>Q, what is the probability for Ivanhoe to lead during the entire voting process ?

If for instance the P votes for him have all been slipped first, he was clearly ahead the whole time. If the very first pirate voted for Isaac, then Ivanhoe was not ahead at this point.
 
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Partial results.
If q = 0, then the probability is 1
If q = 1, then the probability is (p - 1) / (p + 1) because Ivanhoe must receive the first two votes, and after that it doesn't matter when the 1 vote for Isaac occurs. The probability of Ivanhoe receiving the first two votes is:
(p / p + q) (p - 1)/(p + q - 1)
and since q = 1, this is
(p - 1) / (p + 1)
 
Hi,

your partial results are not only correct, but also very close to the general form of the solution :approve:
 
I don't know if anybody is interested in the solution. I did it the hard way, and once I had found the solution I realized there must be a simpler explanation. After browsing the web I finally found that indeed the problem is well known as the "Ballot problem" and even has generalized versions. There is a geometrical proof, direct solution using so-called "Andre's reflection principle", and more.

This is not a spoiler, this is more than a spoiler. The following link contains it, pretty much all :
The Ballot Problem
 

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