[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49 (658ﾚｽ)
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74(1): 現代数学の系譜 雑談 古典ガロア理論も読む ◆e.a0E5TtKE [sage] 2018/01/01(月) 17:12:44.18 ID:dCRrvhl7(10/27) AAS
>>73 つづき

If condition (i) in Theorem 1.1.4 is strengthened to everywhere, the following corollary results.

Corollary 1.1.8. Let J be a σ-ideal which contains no nonempty open set.
A function f : R → R is continuous everywhere if, and only if, it is J -continuous everywhere.

Proof. If f is continuous, then it is clearly J -continuous. So, suppose f is
J -continuous everywhere, x0 ∈ R and ε > 0. Using Proposition 1.1.1(ii), there
must be an ordinary open neighborhood G0 of x0 such that
F0 = {x ∈ G0 : |f(x) − f(x0)| > ε} ∈ J.
Suppose there is an x1 ∈ F0. Choose δ > 0 such that
δ < |f(x1) − f(x0)| − ε.
As before, there exists an ordinary open neighborhood G1 ⊂ G0 of x1 such that
F1 = {x ∈ G1 : |f(x1) − f(x)| > δ} ∈ J.
It is clear that G1 ⊂ F0 ∪ F1 ∈ J, because |f(x1) − f(x0)| > ε + δ. But, this
implies J contains a nonempty open set, which contradicts the condition placed
on J in the statement of the corollary. This contradiction shows that F0 = Φ.
The preceding corollary demonstrates that global J -continuity may not be a
very useful concept. In particular, it is worthwhile noting for future reference
that global I-continuity and global N-continuity are no different than ordinary continuity.

(引用終り)

つづく

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76(1): 現代数学の系譜 雑談 古典ガロア理論も読む ◆e.a0E5TtKE [sage] 2018/01/01(月) 17:13:34.44 ID:dCRrvhl7(12/27) AAS
>>74 つづき

（上記の関連参考：出典URL）
http://www.math.wvu.edu/~kcies/
Krzysztof Chris Ciesielski, Ph.D. Professor of Mathematics at Department of Mathematics, West Virginia University and Adjunct Professor at Medical Image Processing Group, Dept. of Radiology, Univ. of Pennsylvania.
(抜粋)
Books:
(with L. Larson and K. Ostaszewski) I-density continuous functions, Memoirs of the AMS vol. 107 no 515, 1994; MR 94f:54035.
(引用終り)

https://www.amazon.co.jp/I-Density-Continuous-Functions-American-Mathematical/dp/0821825798
I-Density Continuous Functions (Memoirs of the American Mathematical Society) (英語) Krzysztof Ciesielski (著),? Lee Larson (著),? Krzysztof Ostaszewski (著) 1994/1/1

http://www.jstor.org/stable/44151978?seq=1#page_scan_tab_contents
JOURNAL ARTICLE I-density Continuous Functions Krzysztof Ciesielski, Lee Larson and Krzysztof Ostaszewski Real Analysis Exchange Vol. 15, No. 1 (1989-90), pp. 13-15 Published by: Michigan State University Press
（終り）

つづく

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