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What is ICM?
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Shuffle the players' chips. Rank the players by their highest chips. Equivalently, remove the chips from the table one at a time, eliminating a player when his last chip is removed.
Independent Chip Model calculators:
http://www.chillin411.com/icmcalc.php
http://sharnett.bol.ucla.edu/ICM/ICM.html (not as convenient, but it has some explanations)
You can write the formulas for n players by summing over the possible first place finishers, removing their chips, and applying the ICM to the reduced tournament on n-1 players. This doesn't seem to simplify. Does anyone know what the probabilities are of finishing last in a tournament where the stack sizes are 1, 2, ..., 100?
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Is it random walk (brownian motion) on a simplex with absorbing boundaries? I saw Tom Ferguson wrote an exact solution for n=3 players. What else is known?
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The Brownian motion model is different. It's simple to analyze with 3 players because conformal transformations in two dimensions are understood and preserve Brownian paths, and the Riemann maps from the triangle to the disc are even known explicitly. Conformal transformations in higher dimensions are much more rigid, and there isn't much hope to extend the solution to more players without adding ideas.