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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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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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 ...
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Recurrence Relation Solving Problem

Can anyone help me in solving this complex recurrence in detail? $T(n)=n + \sum\limits_{k-1}^n [T(n-k)+T(k)] $ $T(1) = 1$. We want to calculate order of T. I'm confused by using recursion tree ...
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Total function vs Partial function

I'm reading a lecture note in which the following functions ($fact_i : \mathbb{Z}_\perp → \mathbb{Z}_\perp$) for $i \in \mathbb{N}$ are NOT considered total: \begin{equation} fact_0(x) = \perp ...
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Prove/disprove questions on equivalence relations and ordered sets

If $R$ is an equivalence relation and a partial order over $A \neq \emptyset$ then every equivalence class contain at least one element. If $(A,\le)$ an ordered set, and $a\in A$ is a single ...
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If a net is too big, convergence tells us very little?

I'm just learning about nets. It occurs to me that sometimes $x_{\alpha} \to x$ doesn't tell us much at all. Consider the directed set $J:= \{(U,x)\in \mathcal{P}(X) \times X: x \in U \}$, $(U,x) ...
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Is every continuous function measurable?

In non-Hausdorff topology it is standard to define the Borel algebra of a topological space $X$ as the $\sigma$-algebra generated by the open subsets and the compact saturated subsets. Recall that a ...
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Describing orders on sets $\{1,2,3,4\}$ and $\mathbb N$ with minimal/maximal/maximum/minimum elements

Describe all the partial orders on $\{1,2,3,4\}$ where the set of minimal elements are $\{2,4\}$ and the set of maximal elements is $\{1,3\}$ Describe all the partial orders on $\{1,2,3,4\}$ ...
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Has anyone used the isomorphism with $\Bbb{N}_{\gt 0}$ as a monomial ordering?

Let $R[x_1, x_2, \dots]$ be a ring of formal polynomials in a countably $\infty$ number of indeterminates $x_i$, over a commutative ring $R$. The commutative monoid $X = \{ x^e = x_1^{e_1} x_2^{e_2} ...
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What is the meaning of $<$ in a preorder?

Let $(P,\le)$ be a preorder, i.e. $P$ is a set and $\le$ is a relation on it that is reflexive and transitive. In this context for myself I can find two interpretations for the symbol $<$ 1) ...
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Terminology in forcing

In the context of forcing one reads the relation $p \leq q$ in a poset $P$ as "$p$ extends $q$". A typical example is the poset $P$ of finite partial functions, where one defines $p \leq q$ when $q ...
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Has an order with this property a special name?

If $a$ is an element in a preorder then you can eventually go 'a step back' (and repeat this) in the sense of finding an element with $b\leq a$ and not $a\leq b$. Is there a special name for ...
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Every admissible ordering on a countably infinite monoid is induced by an isomorphism onto $(\Bbb{N}, \cdot)$.

Let $fg$ mean the functional composition of functions $f, g$. Let $M$ be a countably infinite, commutative, multiplicative monoid and let $f : M \to (\Bbb{N}_{\gt 0}, \cdot)$ be an isomorphism. ...