Order theory deals with properties of orders, usually partial orders or quasi orders but not only those. Questions about properties of orders, general or particular, may fit into this category, as well as questions about properties of subsets and elements of an ordered set.

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A partial order with more properties than would be expected

Consider the relation: $$\langle x_1,x_2\rangle\prec\langle y_1,y_2\rangle\iff x_1,x_2,y_1,y_2\in\Bbb N\wedge x_1y_2<x_2y_1.$$ This is usually used for defining the (positive) fractions $\Bbb ...
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How to define an isomorphism between $^\omega\omega$ and $\omega^\omega$?

Let $^\omega\omega$ be the set of all functions $x: \omega \to \omega$. Define $A = \{x \in ^\omega\omega \; | \; x \text{ has finite support}\}$, where by "finite support" I mean that the set $\{x(n) ...
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Order Preserving Isomorphism

If abelian group G has an archimedean order then there is an order preserving isomorphism $\phi$ of G onto a subgroup of $\mathbb{R}$. Here we can say that G is archimedean totally ordered abelian ...
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Bijection from $\mathbb{R}^n$ to $\mathbb{R}$ that preserves lexicographic order?

Is there a bijective mapping $f \colon \mathbb{R}^n \rightarrow \mathbb{R}$ that preserves lexicographic order? That's to say, we'd need to have $f(x_1, \dots, x_n) \leq f(y_1, \dots, y_n)$ iff ...
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Existence of certain uncountable closed sets in the order topology

This is a proof-verification request. Let $\Omega$ be the set of countable ordinals, $\omega_1$ the first uncountable ordinal, and $\Omega^*=\Omega\cup\{\omega_1\}$. Remarkable properties of these ...
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Proof on Dyadic Trees [Smullyan: First-Order Logic, chapter 1, section 0]

I'm having difficult with a proof from Smullyan's First-Order Logic, Chapter 1 Section 0 (Reprint, Dover 1968, p. 4): Prove: In a dyadic tree, define x to be to the left of y if there is a ...
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Does a discrete semilattice of finite width have finite number of points between any pair of elements.

Let $(S,<)$ be a countably infinite semilattice such that: 1) $S$ is no-where dense - i.e. there does not exist a subset $T$ such that for all $a,b\in T$ with $a<b$, there exists $c\in T$ with ...
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Non-monotonic functions on ordered sets

I'm trying to prove that if $~~(A,<_A)~~$ and $~~(B,<_B)~~$ are linearly ordered sets and $~~f: A \rightarrow B~~$ is non-monotonic function than there exist points $~~a,b,c\in A~~$ such that ...
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comparing two sets in set theory

I have two sets A and B and this condition holds: $\forall x \in A , y \in B: x \leq y$ Is there any standard term to describe the relation of A and B? something like $A \leq B$? Thanks for your ...
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Undistinguishable elements in posets

Given a finite partially ordered set $P = (V, <)$, I say that $x$ and $y$ in $V$ are indistinguishable in $P$ if for all $z \in V \backslash \{x, y\}$, I have $z < x$ iff $z < y$, and $x < ...
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Definition in Kunen

In Kunen's second edition of set theory he gives the following definition Let $(\mathbb{Q},\leq_\mathbb{Q},\mathbb{1}_\mathbb{Q})$, and $(\mathbb{P},\leq_\mathbb{P},\mathbb{1}_\mathbb{P})$ be forcing ...
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Is a complete flag well-ordered?

Given a vector space $V$, a collection $\mathcal{F}$ of subspaces of $V$ is called a flag of $V$ if $\{0\}\in\mathcal{F}$ $V\in\mathcal{F}$ $\mathcal{F}$ is a chain Furthermore, a flag ...
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How to make an order isomorphism

Two linear orders $A$ and $B$ have starting points $a_0$ and $b_0$, and have cofinalities $\omega_1$. Let $(a_\alpha )_{\alpha<\omega_1}$ and $(b_\alpha )_{\alpha<\omega_1}$ be cofinal ...
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How to determine distribution

I hope you will be patient with the inarticulate question of a non-mathematician. It's hard to get an answer when you don't even know how to ask the question. But here goes... ...Actually, I have two ...
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Name the property $f(x) \ge x$

It's a really one of the simplest properties you could imagine for a function. But I haven't been able to find a name for it. What do you call a function $f$ with the following property: $$f(x) \ge ...
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Lower bounded lattice to complete lattice

My problem is to show that any lower-bounded lattice satisfying the maximal condition is a complete lattice. Let's call the lattice $L$. I'm having some trouble with this. I have tried to look at it ...
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Profinite completion of a partial order

In Johnstone's Stone Spaces it is proved that the category of profinite partial orders is (equivalent to) the category of ordered Stone spaces (also called Priestley spaces) and that the obvious ...
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Suprema always exist $\iff$ Infima always exist

Let $X$ be a poset (or only preordered or even just equipped with a plain relation). Is it true that suprema always exist iff infima always exist: $$\left(\forall A\subseteq X: \sup A\text{ ...
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Proof that order-isomorphisms are bijective

I'm reading the Davey and Priestley's Introduction to Lattices and Order. On page 3, it is said that order-isomorphism is necessarily bijective. I thought we could easily prove the claim given the ...
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Set theory, seems like a difficult question (Choice Function)

So, I don't know if this certain type of set has a name in English. If it has, I'll be glad if someone will edit my post. Definition: Let $A$ be a set and $f: P(A)\setminus \emptyset \rightarrow A$ ...
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Defining principal elements of every poset. Is this a new idea?

Fix an arbitrary complete lattice $\mathfrak{A}$ with order $\sqsubseteq$. I call elements $a,b\in\mathfrak{A}$ intersecting and denote $a\not\asymp b$ iff there is a non-least element ...
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Cardinality of partial orders on $\mathbb R \times \mathbb R$

Let the set $A$ be defined: $A=\{X \subseteq \mathbb R \times \mathbb R: X$ is a partial order $\land \max X \in \mathbb Z \land min X \in \mathbb Z \}$ What's the cardinality of $A$?
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If an infinite well-ordered set has initial segments of finite cardinality only, is the set isomorphic to $\mathbb N$?

Let $A$ be an infinite well-ordered set. Every initial segment of $A$ is finite. Is $A$ isomorphic to $\mathbb N$? What's the way to think about it? Should I build an explicit isomorphism? What ...
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Understanding first and minimal elements

Give an example of a set with partial order, that has a single minimal element but no first element. The example that was given is $\mathbb Z \cup \{2.5\}$, with relation $<$. I can't see why the ...
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increasing subset of a partial order and characteristic function

Can someone help me understand this? Suppose that $\preceq$ is a partial order on a set $S$ and that $A\subseteq S$. If $\mathbf{1}_A$ is the indicator function then $A$ is increasing if ...
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Is (total-)order a weak version of countability?

One can easily see that countability also imposes or enables an ordering of the set. Now $\mathbb{R}$ is ordered but not countable. Is this ordering a weaker version of countability (at least in ...
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compactness and maximal elements

Let $C$ be a nonempty compact subset of $R^n$, with a certain partial order defined on it. I am trying to prove that $C$ contains a maximal element. My idea is: start with a certain element of $C$. ...
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Compactness and existence of Pareto-efficient cake partitions

I am trying to understand a fundamental statement in the theory of cake-cutting. BACKGROUND: There is a certain "cake" $C$ (a subset of $R^n$). The cake is divided among two agents, 0 and 1. Each ...
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Antisymmetric relation (“strong” vs “weak”)

Defining: "weak antisymmetric relation": $\forall a, b \left< a,b \right> \in R \land \left< b,a \right> \in R \Rightarrow a=b$ "Strong antisymmetric relation": $\forall a, b \left< ...
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Are surjective order-homomorphisms necessarily complete lattice homomorphisms?

Let $X$ and $Y$ denote complete lattice, and suppose $f : X \rightarrow Y$ is a surjective order-homomorphism. Does $f$ necessarily preserve arbitrary suprema, therefore being a complete lattice ...
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Order Type of $\mathbb Z_+ \times \{1,2\}$ and $\{1,2\} \times \mathbb Z_+$

I'm currently working on §10 of "Topology" by James R. Munkres. I've got a problem with task 3: Both $\{1,2\} \times \mathbb Z_+$ and $\mathbb Z_+ \times \{1,2\}$ are well-ordered in the ...
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Divisibilty as relation set on $(\mathbb N \setminus \{0,1\})$

So i have to see if $\prec$ is order relation where two elements $(a,b)$ and $(c,d)$ are in relation $\prec$ if $a|c$ and $2b^{2}+6b\leq2d^{2} + 6 d$. This relation is defined on set $(\mathbb N ...
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Principal order filters on a POSET

I have another problem, but in this one I have no idea how to start. Let be $(X,\leq)$ a POSET with a first element and gifted with the topology $\{ (a,\rightarrow) : a \in X \}$ (principal order ...
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Greatest lower and least upper bounds for a set of pairs

Had some trouble with this question in an exam recently, and wanted to make sure I reasoned correctly. The question was: $X$ is a set of pairs of real numbers $(x,y)$, with absolute values less than ...
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Is this proof legal?

Let $\left(P,\le\right)$ denote a poset. Statement: if every sequence $p_{1}\leq p_{2}\leq\cdots$ in $P$ stabilizes (in the sense that for some $n$ we have $k>n\Rightarrow p_k=p_n$) then every ...
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Why are frames called “frames”?

Definition: A frame $F$ is a suplattice such that for any $x_{i}, y\in F$ (for $i\in I$, $I$ a set), we have $$y\wedge\left(\bigvee_{i\in I}x_i\right)=\bigvee_{i\in I}(y\wedge x_i).$$ Why are ...
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Left & right adjoints in the context of complete lattices.

This is a follow-up question from this question of mine. In the same paper as the one mentioned in my previous post, it's stated that In the context of complete lattices, a monotone map has a ...
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Left & right adjoints in the context of posets.

Definition 1: A function $\theta: X\to Y$ between posets is monotone if whenever $x\le y$, we have $\theta (x)\le\theta (y)$. Definition 2: For any pair $f:A\to B$ and $g:B\to A$ of monotone maps, we ...
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Showing a set is well-ordered

Let $(C,S)$ be a well-ordered set. Let $d \notin C$. We define the set $D=C \cup \{d\}$ and the relation $S'=S\cup (C \times \{d\})$. Show the set $(D,S')$ is well-ordered. Any help would be much ...
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Are all preorders implication relations?

This question is for the people who know what consequence relations are. Just to clarify, a consequence relation C on L is a relation from the powerset of L, to L, that satisfies certain properties. ...
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Proving that $\overline{([a,b],\le)}=\overline{([c,d],\le)}$, i.e., the order types of two closed intervals are the same

Prove that for all $a,b,c,d : a\le b : c\le d$ we have 1.$\overline{[a,b]}=\overline{[c,d]},$ 2.$\overline{(a,b)}=\overline{(c,d)}$ 3.$\overline{[a,b)}=\overline{[c,d)}$ ...
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Show that If $C$ is a chain in $X$ then $f(C)$ is also chain in $Y$.

Let X and Y are poset and $f:X\to Y$ is increasing function. If $C$ is a chain in $X$, show that $f(C)$ is also chain in $Y$. Since C is chain for every $x,y \in C: (x,y)\to \left(x\leq y\bigvee ...
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How to prove that $x\leq f(x)$ if f order isomorphic?

Let X is a well ordered set and $f:X\to f(X)=Y\subseteq X$ is order isomorphic. For any $x\in X$ prove that $$x\leq f(x)$$ since X is well ordered, {x,f(x)}$\subseteq$X and $x\leq f(x) $ or ...
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How to assign ranks to members of a partially ordered set with the cardinality of the real numbers

I'm trying to define an informal procedure for assigning ranks to members of a partially ordered set. I know how to do this by an inductive procedure when the poset involved has a finite number of ...
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Coding Forcing Notions by Ordinal Numbers: A Possible Approach to Shelah-Foreman-Magidor Conjecture

Forcing notions are partial orders. In some sense each partial order is a "combination" of some well-orderings and each well-orderings is isomorphic to a unique ordinal number. Thus in some sense a ...
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Prove/disprove questions on isomorphism and embedding between order types

About the notations: Let $\lambda, q, z, \omega$ be the order types of the reals, rationals, integers and natural numbers respectively. The sign $=$ means there's isomorphism and $\le$ means ...
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Understanding Recurrence Relation

as i ask question and answered by some Clever people at this topic: Recurrence Relation Solving Problem i try to learn new thing with new question very similar to get familiar with recurrence ...
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Have you seen this property of tolerance relations before?

Let $A$ be a set equipped with a binary reflexive and symmetric relation $\uparrow$ (such relations are often called tolerances, see also "Are there real-life relations which are symmetric and ...
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Is the series $\frac{1}{(n+1)^p}-\frac{1}{(n-1)^p}$ where 0<p<1 convergent or divergent?

Sorry for my bad English. I really suspect it is convergent. But I can't prove it. Since ${x^p}$ is not derivable at x=0, I can't using taylor expansion to find the order of infinitesimal, thus ...
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Is there a mistake in this question: $\forall a\in A: |\{x\in A:x\le a \}|=|\{ y\in B :y\le a \}|$?

Two ordered sets $(A,\le_A), (B,\le_B)$ and there's an isomorphic function $f:A\to B$ Prove $\forall a\in A: |\{x\in A:x\le a \}|=|\{ y\in B :y\le a \}|$ I think there's a mistake in this ...