Professor Paradoxicus has invented a new game. A single card is dealt to each of two players. The cards are consecutive and come from a 9 card deck, each card bearing a distinct integer between 1 and 9. Each player then holds his card (number facing outward) against his forehead; ie, each player can see his opponent’s card, but not his own. The players are then asked whether they want to bet, and if both agree, they examine their own card, with the player who drew the lower card paying his opponent the amount on the higher card.

Professor Paradoxicus has invited three students to analyse the game.

Simplicimus notes that the game is symmetric and zero sum. He asserts that players will be indifferent as to whether or not to bet.

Optimisticus points out that whenever a player sees his opponent holding a 9 he can count on losing. On the other hand, Optimisticus reasons, if the opponent is holding any card N<9, then that player has a 50% chance of winning N+1 and a 50% chance of losing N. Optimisticus asserts that a player will quit when he sees his opponent holding a 9, and choose to play otherwise.

Finally, Sceptisimus expresses the view that a player will refuse to bet unless he sees his opponent holding a 1. He justifies his peculiar opinion as follows: Suppose a player sees his opponent holding a 9. He will certainly refuse to bet. Now suppose a player sees his opponent holding an 8. Clearly, he himself will be holding either a 9 or a 7. In the former case, he can be sure that his opponent will refuse to bet. He concludes that he has nothing to gain by agreeing to bet, but could have something to lose. He therefore should refuse to bet.

Sceptisimus next reapplies this argument repeatedly, with 7 (,6,5..) taking the role of 8 (,7,6..).

What’s your view of these three arguments and how do things change if the players, while still aware that the deck is finite, don’t know what are the lowest and highest numbers?

(In reply to Zero Sum Confusion by Steve Herman)

b) Therefore, it is false to say that "two rational players will choose only to gamble if the game is zero-sum".  Consider a non-zero sum game where you and I play, and each of us wins a dollar every time.  I'm sure both of us would be willing to play this game.

If the game is non-zero sum between two players, one of the players MUST have an advantage over the other.  The player without the advantage, being rational, would choose NOT to play.  Therefore, two rational players would not choose to gamble unless the game was symmetric and zero sum.  If we have a third party staking, in part or whole, the possible winnings, then there would be three players, not two.

There is no confusion about zero sum.

Edited on February 19, 2008, 5:41 pm

Posted by Dej Mar on 2008-02-18 22:49:12