現代数学の系譜カントル超限集合論他 3

[過去ﾛｸﾞ] 現代数学の系譜カントル超限集合論他 3 (548ﾚｽ)
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39: １３２人目の素数さん [sage] 2020/07/27(月) 14:54:54.76 ID:dppBRBhf

>>37
補足

Frechet filterの英wikipedia記事と
”Examples
On the set N of natural numbers, the set of infinite intervals B = { (n,∞) : n ∈ N} is a Frechet filter base, i.e., the Frechet filter on N consists of all supersets of elements of B.”
あと、MathWorld
”Cofinite Filter
If S is an infinite set, then the collection F_S={ A ⊆ S:S-A is finite} is a filter called the cofinite (or Frechet) filter on S.”

（参考）
https://en.wikipedia.org/wiki/Fr%C3%A9chet_filter
Frechet filter
(抜粋)
In mathematics, the Frechet filter, also called the cofinite filter, on a set is a special subset of the set's power set. A member of this power set is in the Frechet filter if and only if its complement in the set is finite. This is of interest in topology, where filters originated, and relates to order and lattice theory because a set's power set is a partially ordered set (and more specifically, a lattice) under set inclusion.

The Frechet filter is named after the French mathematician Maurice Frechet (1878-1973), who worked in topology. It is alternatively called a cofinite filter because its members are exactly the cofinite sets in a power set.

Contents
1 Definition
2 Properties
3 Examples
4 See also
5 References

Examples
On the set N of natural numbers, the set of infinite intervals B = { (n,∞) : n ∈ N} is a Frechet filter base, i.e., the Frechet filter on N consists of all supersets of elements of B.[citation needed]

External links
・Weisstein, Eric W. "Cofinite Filter". MathWorld.
https://mathworld.wolfram.com/CofiniteFilter.html
Cofinite Filter
If S is an infinite set, then the collection F_S={ A ⊆ S:S-A is finite} is a filter called the cofinite (or Frechet) filter on S.

http://rio2016.5ch.net/test/read.cgi/math/1595034113/39

42: 現代数学の系譜 雑談 ◆yH25M02vWFhP  [sage] 2020/07/27(月) 21:41:57.91 ID:slbIBvLt

>>39 補足
https://arxiv.org/pdf/1212.5740.pdf
Filters and Ultrafilters in Real Analysis 2012
Max Garcia Mathematics Department California Polytechnic State University

Abstract
We study free filters and their maximal extensions on the set of natural numbers.
We characterize the limit of a sequence of real numbers in terms of the Fr´echet filter, which involves only one quantifier as opposed to the three non-commuting quantifiers in the usual definition.
We construct the field of real non-standard numbers and study their properties.
We characterize the limit of a sequence of real numbers in terms of non-standard numbers which only requires a single quantifier as well.
We are trying to make the point that the involvement of filters and/or non-standard numbers leads to a reduction in the number of quantifiers and hence, simplification, compared to the more traditional ε, δ-definition of limits in real analysis.

Contents
Introduction .  . 1
1 Filters, Free Filters and Ultrafilters 3
1.1 Filters and Ultrafilters . . .. 3
1.2 Existence of Free Ultrafilters . . . . . . 5
1.3 Characterization of the Ultrafilter . . . . . . 6
2 The Fr´echet Filter in Real Analysis 8
2.1 Fr´echet Filter . . . . .  . . . . 8
2.2 Reduction in the Number of Quantifiers . . .. . . 10
2.3 Fr´echet filter in Real Analysis . . . . . . . 11
2.4 Remarks Regarding the Fr´echet Filter . .  . . . 12
3 Non-standard Analysis 14
3.1 Construction of the Hyperreals *R . . . . . 14
3.2 Finite, Infinitesimal, and Infinitely Large Numbers . . . . . . . 16
3.3 Extending Sets and Functions in *R . . . . . . . . . . . . . . . 20
3.4 Non-Standard Characterization of Limits in R . . . . . . . . . 23
A The Free Ultrafilter as an Additive Measure 25

http://rio2016.5ch.net/test/read.cgi/math/1595034113/42

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