ch3cooh: (light on black water)
[personal profile] ch3cooh
I'm about to teach a summer math course: Proofs and Visualizations (it's gonna be awesome!) and I've been talking about the problems/units with a bunch of friends.  One of the units is based on a math puzzle called "The Paradox of Perfectly Logical Pirates"  I've outlined the problem below.  However, here's the glitch - once you've come up with the straight-forward answer, here's another question:  Two twin brothers, Alfie and Ben are given $10 of allowance every week.  More specifically, Alfie's given the money and his father tells him to split it between himself and his brother Ben.  As long as Ben accepts the split, the boys get to keep the allowance, but if Ben ever complains, neither boy will get anything.  Alfie and Ben are logical kids, and both realize that something is better than nothing.  So, each week, Alfie gives Ben $1 and keeps $9 for himself.  And Ben doesn't complain because $1 is better than nothing.  But then, one day, Ben goes over to his friend David's house.  David's twin sister, Kate, has just been given $10 allowance to split between herself and David - their father has the exact same rules.  Perhaps David is more sure of himself or something, because he's decided that he'll reject the split unless his sister Kate gives him $5.  Since Kate wants to keep getting allowance ($5 is better than nothing) - she splits the money evenly between them each week.  Ben sees this and is surprised and impressed.  Can he get the same results from his brother Alfie?  Later that week, Ben visits his friend Fanny's house, although Ben doesn't like Fanny all that much - she's  a bit greedy and reminds Ben of his brother Alfie.  When Fanny's brother Elmo gets $10 of allowance to split between the two of them, Fanny tells Elmo that she'll complain to father unless Elmo gives her $9.  Elmo believes her (she is a greedy one) and would rather get $1 than nothing, so each week he gives Fanny $9.  Sometimes Ben wonders if he could get Alfie to do this for a while at least - just in order to pay him back for the many weeks of taking $9 for himself. 

In real life, who has the power? - the person splitting the money, or the most stubborn and greedy player? Why?  How can this question be described rigorously mathematically? 

The Paradox

100 pirates crew a pirate ship that has just robbed a merchant vessel of 1000 gold coins.  Each pirate on the ship has a public rank: Captain, First Mate, Second Mate, Third Mate, ... the 99th Mate who cleans the parrot poo off the deck.  But the pirates have a democratic system set up for dividing the loot.  The captain proposes a plan which defines how the 100 pieces are to be divied up among all pirates, including himself and then the pirates vote on the plan, each pirate including the captain getting one vote.  If half or more of the pirates approve the plan, then it goes into action - problem solved!  But if more than half of the pirates vote the plan down, then the captain is killed and each pirate shifts up one rank.  (The first mate becomes captain, the second mate becomes 1st mate, etc.)  and the new captain is responsible for proposing his own plan for distributing the loot among the 99 remaining pirates.  Here's what all of the pirates know about themselves and every other pirate:
1) Above everything else, a pirate will prioritize saving his own life
2) Above everything except his life, a pirate will prioritize maximizing the amount of gold he gets down to the last coin
3) If he's ensured his survival, and if he's gotten as much gold as he can get, a pirate would rather see a man killed than not - it's entertaining!

How does the captain divide the loot?

Complication:  If you solve the problem above with logic, starting with the case of 1 pirate (aka, everyone's dead besides the original 99th mate), then building up to 2, then 3, then eventually 100, you should get a solution in which the 1st mate doesn't get any gold, and most other pirates don't get very much if anything at all.  The first mate, we'll call him Ben for consistency, thinks he could do better for himself with a bold strategy - he goes around making very solemn promises to the other pirates that he'll give them more than the current captain if they vote the captain's plan down.  The other pirates don't believe him at first, but then they realize that, since he's getting nothing right now, it actually is in his advantage to go through with a fairer plan, so that he at least gets something.  However, they also realize that once they mutany, there's no way to hold the first mate to his promise.  Both effects seem logical.  What happens?


I'm really thrown by these puzzles/'paradoxes' and I feel like there's got to be a good theory somewhere to explain what's going on.  I also think these questions are practically relevant since they suggest that things like mutinies might have a logical basis, whereas simple economic theory frequently predicts that the masses will just accept their short-changed fate. 
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