>>659 (引用開始) >>多分、下記のような日本語「二項関係が整礎（せいそ、英: well-founded）であるとは、真の無限降下列をもたないことである」が、ミスリードです >「空集合Φより簡単な集合はない」を公理にしたのが、正則性公理です 1．下記 wikipedia 正則性公理の説明にも、「∀xについて、無限下降列である x∋x1∋x2∋... は存在しない」が出てきますが 繰り返しますが、ダメなのは、「”xn+1 R xn”なる ”countable infinite descending chains”」(>>651)なのです 逆の「x∈x1∈x2∈... 」なる無限列はOKです。勘違いしているサル二匹がいます (引用終り)
補足説明しておこう 1．問題の”無限下降列”では、下記英文 Well-founded relationの ”Equivalently, assuming the axiom of dependent choice, a relation is well-founded if it contains no countable infinite descending chains: that is, there is no infinite sequence x0, x1, x2, ... of elements of X such that xn+1 R xn for every natural number n.” が最も正確な表現なのです 繰り返すが、”there is no infinite sequence x0, x1, x2, ... of elements of X such that xn+1 R xn for every natural number n.”ね 2．ここ、infiniteでなく、有限だと意味が微妙です 例えて言えば、いま目の前に階段があるとする。下りが上りか？ 自分の立ち位置で違う。下から見れば上りで、上から見れば下り つまり、有限なら、一つの階段に対して、どちらの見方もありうる しかし、エンドレスの無限階段なら？ どちらか一つしかあり得ない。エンドレスだから、逆からの見方はできない。無限に上るか、無限に下るかしかないのです 3．日常語の感覚のまま、「無限降下列」を考えて、”どちらの見方もありうる”！ とハマル おサルがいます（＾＾；
（参考） https://en.wikipedia.org/wiki/Well-founded_relation Well-founded relation In mathematics, a binary relation R is called well-founded (or wellfounded) on a class X if every non-empty subset S ⊆ X has a minimal element with respect to R, that is, an element m not related by sRm (for instance, "s is not smaller than m") for any s ∈ S.
Equivalently, assuming the axiom of dependent choice, a relation is well-founded if it contains no countable infinite descending chains: that is, there is no infinite sequence x0, x1, x2, ... of elements of X such that xn+1 R xn for every natural number n.
https://ja.wikipedia.org/wiki/%E6%95%B4%E7%A4%8E%E9%96%A2%E4%BF%82 数学において、二項関係が整礎（せいそ、英: well-founded）であるとは、真の無限降下列をもたないことである。 定義 集合あるいはクラス X 上の二項関係 R が整礎であるとは、X の空でない任意の部分集合 S が R に関する極小元を持つことをいう[1]。 X が集合であるとき、従属選択公理（英語版）（これは選択公理よりも真に弱く可算選択公理よりも真に強い）を仮定すれば、同値な定義として、関係が整礎であることを可算無限降下列が存在しないこととして定められる[3]。つまり、X の元の無限列 x0, x1, x2, ... で、どんな n についても xn+1 R xn となるようなものはとれない。
>>748 補足 (引用開始) 1．問題の”無限下降列”では、下記英文 Well-founded relationの ”Equivalently, assuming the axiom of dependent choice, a relation is well-founded if it contains no countable infinite descending chains: that is, there is no infinite sequence x0, x1, x2, ... of elements of X such that xn+1 R xn for every natural number n.” が最も正確な表現なのです 繰り返すが、”there is no infinite sequence x0, x1, x2, ... of elements of X such that xn+1 R xn for every natural number n.”ね 例えて言えば、いま目の前に階段があるとする。下りが上りか？ 自分の立ち位置で違う。下から見れば上りで、上から見れば下り つまり、有限なら、一つの階段に対して、どちらの見方もありうる しかし、エンドレスの無限階段なら？ どちらか一つしかあり得ない。エンドレスだから、逆からの見方はできない。無限に上るか、無限に下るかしかないのです (引用終り)
現代数学では、”無限”の意味が多様化してしまった 本来は、「限りが無い」＝”無限”だった 英語でも、finite の語源は、下記のように”L.finire = to end(終わる)”だとか。L.finire は、フィナーレ 【(イタリア)finale】も同様でしょう
下記、英語のInfinity wikipedia などを見ると、 Actual infinity（和訳では「実無限」） と ”potential infinity, in which a non-terminating process (such as "add 1 to the previous number") produces a sequence with no last element, and where each individual result is finite and is achieved in a finite number of steps. ” とに分けて説明しています
https://en.wikipedia.org/wiki/Actual_infinity Actual infinity In the philosophy of mathematics, the abstraction of actual infinity involves the acceptance (if the axiom of infinity is included) of infinite entities as given, actual and completed objects. These might include the set of natural numbers, extended real numbers, transfinite numbers, or even an infinite sequence of rational numbers.[1] Actual infinity is to be contrasted with potential infinity, in which a non-terminating process (such as "add 1 to the previous number") produces a sequence with no last element, and where each individual result is finite and is achieved in a finite number of steps. As a result, potential infinity is often formalized using the concept of limit.[2]
https://en.wikipedia.org/wiki/Infinity Infinity History Early Greek Aristotle (350 BC) distinguished potential infinity from actual infinity, which he regarded as impossible due to the various paradoxes it seemed to produce.[9]
Infinity wikipedia に下記の Wiles's proof of Fermat's Last Theorem と Grothendieck universes の関係が書いてあった これ面白いわ（＾＾；
https://en.wikipedia.org/wiki/Infinity Infinity
The mathematical concept of infinity and the manipulation of infinite sets are used everywhere in mathematics, even in areas such as combinatorics that may seem to have nothing to do with them. For example, Wiles's proof of Fermat's Last Theorem implicitly relies on the existence of very large infinite sets[7] for solving a long-standing problem that is stated in terms of elementary arithmetic.
References [7] McLarty, Colin (2010). "What does it take to prove Fermat's Last Theorem? Grothendieck and the logic of number theory". The Bulletin of Symbolic Logic. 16 (3): 359–377. doi:10.2178/bsl/1286284558. https://www.cambridge.org/core/journals/bulletin-of-symbolic-logic/article/what-does-it-take-to-prove-fermats-last-theorem-grothendieck-and-the-logic-of-number-theory/80EDFF3616F8D58590EBA0DCB9FD2E3E （PDF） https://www.cambridge.org/core/services/aop-cambridge-core/content/view/80EDFF3616F8D58590EBA0DCB9FD2E3E/S1079898600000810a.pdf/what-does-it-take-to-prove-fermats-last-theorem-grothendieck-and-the-logic-of-number-theory.pdf
Abstract. This paper explores the set theoretic assumptions used in the current published proof of Fermat’s Last Theorem, how these assumptions figure in the methods Wiles uses, and the currently known prospects for a proof using weaker assumptions.
Does the proof of Fermat’s Last Theorem (FLT) go beyond Zermelo Fraenkel set theory (ZFC)? Or does it merely use Peano Arithmetic (PA) or some weaker fragment of that? The answers depend on what is meant by “proof” and “use,” and are not entirely known. This paper surveys the current state of these questions and briefly sketches the methods of cohomological number theory used in the existing proof.
The existing proof of FLT is Wiles [1995] plus improvements that do not yet change its character. Far from self-contained it has vast prerequisites merely introduced in the 500 pages of [Cornell et al., 1997]. We will say that the assumptions explicitly used in proofs that Wiles cites as steps in his own are “used in fact in the published proof.” It is currently unknown what assumptions are “used in principle” in the sense of being proof-theoretically indispensable to FLT. Certainly much less than ZFC is used in principle, probably nothing beyond PA, and perhaps much less than that.
The oddly contentious issue is universes, often called Grothendieck universes. 1 On ZFC foundations a universe is an uncountable transitive set U such that U, ∈ satisfies the ZFC axioms in the nicest way: it contains the powerset of each of its elements, and for any function from an element of U to U the range is also an element of U. This is much stronger than merely saying U, ∈ satisfies the ZFC axioms. We do not merely say the powerset axiom “every set has a powerset” is true with all quantifiers relativized to U. Rather, we require “for every set x ∈ U, the powerset of x is also in U” where no quantifier in the definition of the powerset of x is relativized to U. (引用終り) 以上
（参考） https://en.wikipedia.org/wiki/Ordinal_number Ordinal number
https://upload.wikimedia.org/wikipedia/commons/thumb/1/18/Ordinal_ww.svg/384px-Ordinal_ww.svg.png A graphical "matchstick" representation of the ordinal ω2. Each stick corresponds to an ordinal of the form ω・m+n where m and n are natural numbers.
There are infinite ordinals as well: the smallest infinite ordinal is ω, which is the order type of the natural numbers (finite ordinals) and that can even be identified with the set of natural numbers. Indeed, the set of natural numbers is well-ordered?as is any set of ordinals?and since it is downward closed, it can be identified with the ordinal associated with it (which is exactly how {\displaystyle \omega }\omega is defined).
Perhaps a clearer intuition of ordinals can be formed by examining a first few of them: as mentioned above, they start with the natural numbers, 0, 1, 2, 3, 4, 5, … After all natural numbers comes the first infinite ordinal, ω, and after that come ω+1, ω+2, ω+3, and so on. (Exactly what addition means will be defined later on: just consider them as names.)
After all of these come ω・2 (which is ω+ω), ω・2+1, ω・2+2, and so on, then ω・3, and then later on ω・4. Now the set of ordinals formed in this way (the ω・m+n, where m and n are natural numbers) must itself have an ordinal associated with it: and that is ω2. Further on, there will be ω3, then ω4, and so on, and ωω, then ωωω, then later ωωωω, and even later ε0 (epsilon nought) (to give a few examples of relatively small?countable?ordinals). This can be continued indefinitely (as every time one says "and so on" when enumerating ordinals, it defines a larger ordinal). The smallest uncountable ordinal is the set of all countable ordinals, expressed as ω1 or ω.
>>780 (引用開始) ここ、下記の”graphical "matchstick" representation”が、分かり易い "matchstick"は、21世紀では死語かも。後述のマッチwikipediaご参照 https://en.wikipedia.org/wiki/Ordinal_number Ordinal number https://upload.wikimedia.org/wikipedia/commons/thumb/1/18/Ordinal_ww.svg/384px-Ordinal_ww.svg.png A graphical "matchstick" representation of the ordinal ω2. Each stick corresponds to an ordinal of the form ω・m+n where m and n are natural numbers.
There are infinite ordinals as well: the smallest infinite ordinal is ω, which is the order type of the natural numbers (finite ordinals) and that can even be identified with the set of natural numbers. Indeed, the set of natural numbers is well-ordered?as is any set of ordinals?and since it is downward closed, it can be identified with the ordinal associated with it (which is exactly how ω is defined). Perhaps a clearer intuition of ordinals can be formed by examining a first few of them: as mentioned above, they start with the natural numbers, 0, 1, 2, 3, 4, 5, … After all natural numbers comes the first infinite ordinal, ω, and after that come ω+1, ω+2, ω+3, and so on. (Exactly what addition means will be defined later on: just consider them as names.) (引用終り)
集積点（しゅうせきてん、英: accumulation point）あるいは極限点（きょくげんてん、英: limit point）は、位相空間 X の部分集合 S に対して定義される概念。（X の位相に関する x の任意の近傍が x 自身を除く S の点を含むという意味で）S によって「近似」できる X の点 x を S の集積点と呼ぶ。このとき、集積点 x は必ずしも S の点ではない。たとえば実数 R の部分集合 S = { 1/n | n ∈ N } を考えたとき点 0 は S の（唯一の）集積点である。集積点の概念は極限の概念を適切に一般化したもので、閉集合や閉包といった概念を下支えする。実際、集合が閉であることとそれが自身の集積点を全て含むことは同値で、集合に対する閉包作用はもとの集合にその集積点を付け加えることによる拡大操作としても捉えられる。
つまり、>>750の ”無限下降列”（ infinite descending chains）は、下記英文 Well-founded relationの ”Equivalently, assuming the axiom of dependent choice, a relation is well-founded if it contains no countable infinite descending chains: that is, there is no infinite sequence x0, x1, x2, ... of elements of X such that xn+1 R xn for every natural number n.” が最も正確な表現です
この逆の”xn R xn+1”は、上昇列で左から右に順序数が増えていきます 一方、もとの”xn+1 R xn”は、可算無限下降列（ countable infinite descending chains）を表現しています （上記とは逆に、右から左に順序数が増えていきます）
定義 二項関係 R は通常、任意の集合（または類）X, Y とそれらの直積 X × Y の部分集合 G の順序三つ組 (X, Y, G) として定義される。このとき、集合 X および Y はそれぞれこの関係の始集合 (domain) および終集合 (codomain) と呼ばれ、G はこの関係のグラフと呼ばれ、G(R) と表すこともある。
R が関係 (X, Y, G) であるとき、(x, y) ∈ G となることを、「x は y と R-関係を持つ」などといい、x?R?y や R(x, y) で表す。後者は、対の集合 G の指示函数として R を見ることに対応する。
876 名前：要素の順番は重要で、a ≠ b ならば a?R?b および b?R?a はそれぞれ独立に真にも偽にもなりうる。
特殊な二項関係 X と Y 上の二項関係のいくつか重要なクラスを以下に挙げる。 (略)
集合上の関係 X = Y で二項関係の始集合 X と終集合 Y とが一致しているならば、簡単に X 上の二項関係（あるいはもう少し明示的に X 上の自己関係 (endorelation)）と呼ぶ。自己関係のいくつかのクラスについては有向グラフとしてグラフ理論において広く調べられている。 集合 X 上の二項関係全体の成す集合 B(X) は、関係をその逆関係へ写す対合を備えた対合付き半群を成す。 集合 X 上の二項関係のいくつか重要なクラスとして、以下のようなものを挙げることができる：
集合的 (set-like) 集合 X の任意の元 x に対して、y?R?x となるような y 全体の成すクラスが集合であるような関係は、集合的（あるいは集合状、集合様）であるという。 （これは真のクラス上の関係を認める場合でないと意味を持たない） 順序数全体の成すクラス上の通常の順序関係 "<" は集合的関係だが、その逆順序 ">" は集合的ではない。
整礎的 (well-founded) X の任意の空でない部分集合Aが極小元a(Aのどの元xもxRaとならない)を持つときR は整礎的であるという。 自然数上の大小関係"?"は整礎的である。正則性公理を仮定すると∈は任意の集合上で整礎的である。
Properties Some important properties that a homogeneous relation R over a set X may have are:
Set-like[citation needed] (or local) [citation needed] for all x ∈ X, the class of all y such that yRx is a set. (This makes sense only if relations over proper classes are allowed.) For example, the usual ordering < over the class of ordinal numbers is a set-like relation, while its inverse > is not. (引用終り) 以上
https://en.wikipedia.org/wiki/Binary_relation 4 Sets versus classes Sets versus classes Certain mathematical "relations", such as "equal to", "subset of", and "member of", cannot be understood to be binary relations as defined above, because their domains and codomains cannot be taken to be sets in the usual systems of axiomatic set theory. For example, if we try to model the general concept of "equality" as a binary relation =, we must take the domain and codomain to be the "class of all sets", which is not a set in the usual set theory.
In most mathematical con
894 名前：texts, references to the relations of equality, membership and subset are harmless because they can be understood implicitly to be restricted to some set in the context. The usual work-around to this problem is to select a "large enough" set A, that contains all the objects of interest, and work with the restriction =A instead of =. Similarly, the "subset of" relation ⊆ needs to be restricted to have domain and codomain P(A) (the power set of a specific set A): the resulting set relation can be denoted by ⊆A. Also, the "member of" relation needs to be restricted to have domain A and codomain P(A) to obtain a binary relation ∈A that is a set. Bertrand Russell has shown that assuming ∈ to be defined over all sets leads to a contradiction in naive set theory.
Another solution to this problem is to use a set theory with proper classes, such as NBG or Morse?Kelley set theory, and allow the domain and codomain (and so the graph) to be proper classes: in such a theory, equality, membership, and subset are binary relations without special comment. (A minor modification needs to be made to the concept of the ordered triple (X, Y, G), as normally a proper class cannot be a member of an ordered tuple; or of course one can identify the binary relation with its graph in this context.)[20] With this definition one can for instance define a binary relation over every set and its power set. (引用終り) 以上
