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115: １３２人目の素数さん [] 2022/10/26(水) 20:15:56.48 ID:b4wD2Jth

>>114
つづき

懐疑派2 DR Alexander Pruss氏
https://mathoverflow.net/questions/151286/probabilities-in-a-riddle-involving-axiom-of-choice
Probabilities in a riddle involving axiom of choice
asked Dec 9 '13 at 16:16 Denis

<回答者 DR Alexander Pruss氏>
Here's an amusing thing that may help see how measurability enters into these things. Consider a single sequence of infinitely many independent fair coin flips. Our state space is Ω={0,1}^N, corresponding to an infinite sequence (Xi)∞i=0 of i.i.d.r.v.s with P(Xi=1)=P(Xi=0)=1/2. Start with P being the completion of the natural product measure on Ω.

That's a fine argument assuming the function is measurable. But what if it's not? Here is a strategy: Check if X1,X2,... fit with the relevant representative. If so, then guess according to the representative. If not, then guess π. (Yes, I realize that π not∈{0,1}.) Intuitively this seems a really dumb strategy.

つづく

http://rio2016.5ch.net/test/read.cgi/math/1666352731/115

116: １３２人目の素数さん [] 2022/10/26(水) 20:16:18.44 ID:b4wD2Jth

>>115

つづき

懐疑派3 回答者 DR Tony Huynh氏
https://mathoverflow.net/questions/151286/probabilities-in-a-riddle-involving-axiom-of-choice
Probabilities in a riddle involving axiom of choice
asked Dec 9 '13 at 16:16 Denis

<回答者 DR Tony Huynh氏>
I also like this version of the riddle. To answer the actual question though, I would say that it is not possible to guess incorrectly with probability only 1/N, even for N=2. 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.

Here's a concrete choice for a probability space that shows that your proposal will fail. Suppose that for each index i we sample a real number Xi from the normal distribution so that the Xis are independent random variables. If there is only person, no matter which boxes they view, they gain no information about the un-opened boxes due to independence. Thus, their probability of guessing correctly is actually 0, not (N?1)/N, say.

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.
(引用終り)
以上

http://rio2016.5ch.net/test/read.cgi/math/1666352731/116

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