現代数学の系譜工学物理雑談古典ガロア理論も読む47

[過去ﾛｸﾞ] 現代数学の系譜工学物理雑談古典ガロア理論も読む47 (650ﾚｽ)
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51: 現代数学の系譜 雑談 古典ガロア理論も読む ◆e.a0E5TtKE  [sage] 2017/11/30(木) 22:27:46.93 ID:IqNIthYM

>>50 つづき
45 https://rio2016.5ch.net/test/read.cgi/math/1508931882/471
471 現代数学の系譜 工学物理雑談 古典ガロア理論も読む 20171106

で、むしろ時枝記事に近いのは、君が>>295（>>304）で紹介した下記の方が、時枝に近いだろう
ここでは、任意の関数f(x)の任意の貴方の選ぶ１点（”You pick an x ∈ Ｒ”）を、” whatever f Bob picked, you will win the game with probability 1!”、”it’s arbitrary: it doesn’t have to be continuous or anything”の条件で当てられるとあるよ

Ｎ⊂Ｒだから、”You pick an n ∈ Ｎ”とすれば、時枝記事の場合を含むことになろう
で、時枝記事のように、どこの箱が当たるか分らず、また確率99/100に対して、これは自分で選んだｘであり、”with probability 1!”だから、こちらの解法がよほど優れている

https://xorshammer.com/2008/08/23/set-theory-and-weather-prediction/
SET THEORY AND WEATHER PREDICTION XOR’S HAMMER Some things in mathematical logic that I find interesting WRITTEN BY MKOCONNOR Blog at WordPress.com. AUGUST 23, 2008
(抜粋)
Here’s a puzzle:
You and Bob are going to play a game which has the following steps.

1)Bob thinks of some function f： Ｒ → Ｒ (it’s arbitrary: it doesn’t have to be continuous or anything).
2)You pick an x ∈ Ｒ.
3)Bob reveals to you the table of values {(x0, f(x0))｜ x0 ≠ x } of his function on every input except the one you specified
4)You guess the value f(x) of Bob’s secret function on the number x that you picked in step 2.

You win if you guess right, you lose if you guess wrong. What’s the best strategy you have?
This initially seems completely hopeless: the values of f on inputs x0 ≠ x have nothing to do with the value of f on input x, so how could you do any better then just making a wild guess?
In fact, it turns out that if you, say, choose x in Step 2 with uniform probability from [ 0,1 ], the axiom of choice implies that you have a strategy such that, whatever f Bob picked, you will win the game with probability 1!
つづく

http://rio2016.5ch.net/test/read.cgi/math/1512046472/51

52: 現代数学の系譜 雑談 古典ガロア理論も読む ◆e.a0E5TtKE  [sage] 2017/11/30(木) 22:28:15.89 ID:IqNIthYM

>>51 つづき

スレ45 https://rio2016.5ch.net/test/read.cgi/math/1508931882/472
472 自分返信：現代数学の系譜 工学物理雑談 古典ガロア理論も読む[sage] 投稿日：2017/11/06(月) 00:05:26.40 ID:1Au30FRy [6/13]

The strategy is as follows: Let ～ be the equivalence relation on functions from Ｒ to Ｒ defined by f ～ g iff for all but finitely many y, f(y) = g(y). Using the axiom of choice, pick a representative from each equivalence class.

In Step 2, choose x with uniform probability from [ 0,1 ].
When, in step 3, Bob reveals {(x0, f(x0)) ｜ x0 ≠ x }, you know what equivalence class f is in, because you know its values at all but one point. Let g be the representative of that equivalence class that you picked ahead of time. Now, in step 4, guess that f(x) is equal to g(x).

What is the probability of success of this strategy?
Well, whatever f that Bob picks, the representative g of its equivalence class will differ from it in only finitely many places.
You will win the game if, in Step 2, you pick any number besides one of those finitely many numbers.
Thus, you win with probability 1 no matter what function Bob selects.
(引用終り)

http://rio2016.5ch.net/test/read.cgi/math/1512046472/52

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