It is well-known that the axiom of choice is equivalent to many other assumptions, such as the well-ordering principle, Tychonoff's theorem, and the fact that every vector space has a basis. Even though all these formulations are equivalent, I have heard many people say that they 'believe' the axiom of choice, but they don't 'believe' the well-ordering principle.

So, my question is what do you consider to be the most unintuitive application of choice?

Here is the sort of answer that I have in mind.

An infinite number of people are about to play the following game. In a moment, they will go into a room and each put on a different hat. On each hat there will be a real number. Each player will be able to see the real numbers on all the hats (except their own). After all the hats are placed on, the players have to simultaneously shout out what real number they think is on their own hat. The players win if only a finite number of them guess incorrectly. Otherwise, they are all executed. They are not allowed to communicate once they enter the room, but beforehand they are allowed to talk and come up with a strategy (with infinite resources).

The *very* unintuitive fact is that the players have a strategy whereby they can *always* win. Indeed, it is hard to come up with a strategy where at least *one* player is guaranteed to answer correctly, let alone a co-finite set. **Hint**: the solution uses the axiom of choice.

10more comments