Hat Problems

Joe Buhler

SAMPLE PROBLEM A team of three people meet and have the rules of the game below described to them. They then have a strategy session. Afterwards, they are taken into a room, and each person has a random 0 or 1 placed on their forehead (i.e., all 8 placements are equally likely). They can see the numbers on the other two foreheads, but not their own. Then they are each *simultaneously* and *without any communication* required to make one of the following three statements: "there is a 0 on my forehead", "there is a 1 on my forehead", or "I pass". The team collectively shares a one million dollar prize if and only if at least of them doesn't pass, and *all* non-pass statements that are made are true statements. An obvious strategy that gives them a 50/50 chance of collecting the million dollars is for them to agree in the strategy session that Alice will guess randomly and that the others will pass. Is there any strategy that they can choose that gives them a better than 50/50 chance of winning?

The actual topics of the talk are (a) discrete geometry, and (b) probability.