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Probabilities in a riddle involving axiom of choice Dec 9 '13
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asked Dec 9 '13 at 16:16 Denis
I think it is ok, because the only probability measure we need is uniform probability on {0,1,…,N-1}, but other people argue it's not ok, because we would need to define a measure on sequences, and moreover axiom of choice messes everything up.
Alexander Pruss
Let's go back to the riddle. Suppose u￣ is chosen randomly. The most natural option is that it is a nontrivial i.i.d. sequence (uk), independent of the random index i which is uniformly distributed over [100]={0,...,99}.
In general, Mj will be nonmeasurable (one can prove this in at least some cases). We likewise have no reason to think that M is measurable. But without measurability, we can't make sense of talk of the probability that the guess will be correct.
Denis Dec 17 '13 at 15:21
Our choice of index i is made randomly, but for this we only need the uniform distribution on {0,…,n}. It is made independently of the opponent's choice.
Tony Huynh Dec 9 '13 at 17:37
In order for such a question to make sense, it is necessary to put a probability measure on the space of functions f:N→R.
Note that to execute your proposed strategy, we only need a uniform measure on {1,…,N}, but to make sense of the phrase it fails with probability at most 1/N, we need a measure on the space of all outcomes.
The answer will be different depending on what probability space is chosen of course.
If it were somehow possible to put a 'uniform' measure on the space of all outcomes,
then indeed one could guess correctly with arbitrarily high precision,
but such a measure doesn't exist.
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