IBM Research | Ponder This | February 2006 solutions

# Ponder This

## February 2006

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I came up with this problem and solution myself. However a reader points out that this model of poker has been solved before. It is usually attributed to Von Neumann (see Theory of Games and Economic Behavior by Von Neumann and Morgenstern, sections 19.14.1-19.16.2).

We show below that the value of the game is .1.

Let the first player's number be x, the second player's be y. Suppose the second player follows a strategy of calling when y>p and folding otherwise (for some parameter p between 0 and 1). Suppose the first player knows p, then what should he do.

Suppose first x<p.

If the first player does not bet he will win 1 unit a fraction x of the time and lose 1 unit a fraction (1-x) of the time.So his expectation is x-(1-x) or 2*x-1.

If the first player bets he will win 1 unit if not called but lose 2 units if called. So his expectation is p-2*(1-p)=3*p-2.

So the first player should bet if (3*p-2) > (2*x-1) (or x <(1.5*p-.5) ) and not bet otherwise.

Suppose next x>p.

As above if the first player does not bet his expectation is 2*x-1.

If the first player bets he will win 1 unit if not called, 2 units if called with p<y<x but lose 2 units if called with x<y<1. so his expectation is p+2*(x-p)-2*(1-x) or 4*x-p-2.

So the first player should bet if (4*x-p-2) > (2*x-1) (or x >(.5*p+.5) ) and not bet otherwise.

So putting the cases together the first player should bet when x < (1.5*p-.5) or x > (.5*p+.5) and not bet otherwise. Next we compute the first player's expectation.

Suppose first p < 1/3. Then for 0 < x < (.5*p+.5) the first player doesn't bet. His expectation is 2*x-1 which averaged over the interval is .5*p-.5. The interval has length .5*p+.5 so the total expectation for the no bet case is .25*p*p-.25. For .5*p+.5 < x < 1 the first player bets with expectation 4*x-p-2 which averaged over the interval is 1. The interval has length -.5*p+.5 so the total expectation for the bet case is -.5*p+.5. Combining the bet and no bet cases the total expectation is .25*p*p-.5*p+.25.

Suppose next p > 1/3. Here we have 3 intervals. For 0 < x <(1.5*p-.5) the first player bets with expectation 3*p-2. This interval has length 1.5*p-.5 so the total expectation for this interval is4.5*p*p-4.5*p+1. For 1.5*p-.5 < x < .5*p+.5 the first player does not bet with expectation 2*x-1 which has average value 2*p-1 on the interval. This interval has length 1-p so the total expectation for this interval is -2*p*p+3*p-1. Finally for .5*p+.5 < x < 1 the first player bets. As above the total expectation for this interval is-.5*p+.5. Combining the three intervals the total expectation is 2.5*p*p-2*p+.5.

Hence the expectation of the first player is .25*p*p-.5*p+.25(p < 1/3), 2.5*p*p-2*p+.5 (p > 1/3). This expectation is minimized when p=.4 where it has value .1. Hence the value of the game is at most .1 since the second player can limit the value to at most .1 by calling when y>.4 and folding otherwise. The first player's strategy in this case is to not bet for .1<x<.7 and bet otherwise. Suppose the second player knows the first player is following this strategy. The nit is easy to see that if the first player bets the second player should fold if y<.1, is in different between calling and folding for.1<y<.7 and should call when y>.7. So the second player can not do better than calling whenever y>.4 which leaves the first player an expectation of .1. Hence the value of the game is at least .1. This means the value of the game is exactly .1.

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