ガロア第一論文と乗数イデアル他関連資料スレ17

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444: 現代数学の系譜 雑談 ◆yH25M02vWFhP  [] 2025/05/17(土) 23:44:37.10 ID:y2zepp9J

>>442 追加
>「QからRへの連続函数f(x)があるとき、f(x)をRからRへの連続函数に（一意的に）拡張できるか？」では 一様収束は 必要

”Tietze extension theorem”貼っておきますね

(参考)
https://en.wikipedia.org/wiki/Tietze_extension_theorem
Tietze extension theorem

In topology, the Tietze extension theorem (also known as the Tietze–Urysohn–Brouwer extension theorem or Urysohn-Brouwer lemma[1]) states that any real-valued, continuous function on a closed subset of a normal topological space can be extended to the entire space, preserving boundedness if necessary.

Formal statement

Proof
略す
History
L. E. J. Brouwer and Henri Lebesgue proved a special case of the theorem, when
X is a finite-dimensional real vector space. Heinrich Tietze extended it to all metric spaces, and Pavel Urysohn proved the theorem as stated here, for normal topological spaces.[2][3]

Equivalent statements
This theorem is equivalent to Urysohn's lemma (which is also equivalent to the normality of the space) and is widely applicable, since all metric spaces and all compact Hausdorff spaces are normal. It can be generalized by replacing
R with R^J for some indexing set J, any retract of R^J, or any normal absolute retract whatsoever.

https://en.wikipedia.org/wiki/Heinrich_Tietze
Heinrich Tietze
Heinrich Franz Friedrich Tietze (August 31, 1880 – February 17, 1964) was an Austrian mathematician, famous for the Tietze extension theorem on functions from topological spaces to the real numbers. He also developed the Tietze transformations for group presentations, and was the first to pose the group isomorphism problem. Tietze's graph is also named after him; it describes the boundaries of a subdivision of the Möbius strip into six mutually-adjacent regions, found by Tietze as part of an extension of the four color theorem to non-orientable surfaces.

http://rio2016.5ch.net/test/read.cgi/math/1746597368/444

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