21/05/20 07:26:26.93 6qFMF4tQ.net
>>214
ところで
下記のWell-ordering theoremに、
”the well-ordering theorem is equivalent to the axiom of choice, 略 in first order logic .
In second order logic, however, the well-ordering theorem is strictly stronger than the axiom of choice”
とある
これ、面白いね
first order logicと second order logicでは、こんなに違うんだ
日本人で知っている人少ないだろうね（＾＾；
URLﾘﾝｸ(en.wikipedia.org)
Well-ordering theorem
Ernst Zermelo introduced the axiom of choice as an "unobjectionable logical principle" to prove the well-ordering theorem.[3] One can conclude from the well-ordering theorem that every set is susceptible to transfinite induction, which is considered by mathematicians to be a powerful technique.[3]
History
Georg Cantor considered the well-ordering theorem to be a "fundamental principle of thought".[4] However, it is considered difficult or even impossible to visualize a well-ordering of R ; such a visualization would have to incorporate the axiom of choice.[5] In 1904, Gyula K?nig claimed to have proven that such a well-ordering cannot exist. A few weeks later, Felix Hausdorff found a mistake in the proof.[6] It turned out, though, that the well-ordering theorem is equivalent to the axiom of choice, in the sense that either one together with the Zermelo?Fraenkel axioms is sufficient to prove the other, in first order logic (the same applies to Zorn's Lemma). In second order logic, however, the well-ordering theorem is strictly stronger than the axiom of choice: from the well-ordering theorem one may deduce the axiom of choice, but from the axiom of choice one cannot deduce the well-ordering theorem.[7]
There is a well-known joke about the three statements, and their relative amenability to intuition:
The axiom of choice is obviously true, the well-ordering principle obviously false, and who can tell about Zorn's lemma?[8]
略
(引用終り)
以上