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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Properties of infimum and supremum w.r.t. conic orders, in particular positive semidefinite matrices

Given a convex cone $\mathcal{K}\subseteq \mathbb{R}^n$, we can define a partial order $\leq_\mathcal{K}$on $ \mathbb{R}^n$ by setting $$x\leq_\mathcal{K} y \Leftrightarrow y-x\in \mathcal{K}.$$ For ...
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definition of ordered vector space

An ordered vector space is the pair $(V , \leq)$ where it satisfies the following: For all $x,y,z \in V, \lambda \geq 0$, i) $x \leq y \Rightarrow x+z \leq y+z$ ii) $x \leq y \Rightarrow \lambda ...
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Example of $\omega$-complete poset that is not chain complete

An $\omega$-complete poset is a poset in which every countable chain has a join. A chain complete poset is a poset in which every chain has a join. Have you an example of a (non-countable) poset that ...
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A simple way to know whether a well-ordered set has a subset of a certain type

Following my last question, Does $\Bbb R-\Bbb Q$ have a well ordered subset of type $\omega\cdot\omega$, I would like to have better tools to look at a set and know what order types can it have. I ...
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Linear continuum poset with succ and pred

Is there a poset of continuum cardinality that satisfies conditions: it is linear there is a minimum element there is a maximum element each element (apart from maximum) has a successor each element ...
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The order isomorphism theorem without Replacement

In $\sf ZF$, there is the following theorem: Theorem: For any well-ordered set $\langle A,\prec\rangle$, there is a (unique) order isomorphism $F$ from $\langle\alpha,\in\rangle$ to $\langle ...
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What's the name of this type of a set?

So I have a set $\{i_1,i_3,i_5\}$. What do we call the following set? Is there a standard name for it? $\emptyset, \{i_1\}, \{i_1,i_3\}, \{i_1,i_3,i_5\}$. Note that we do not have $\{i_3,i_5\}$ in it ...
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Prob. 9, Sec. 10 in Munkres' TOPOLOGY, 2nd ed: How to verify transitivity for anti-dictionary order?

Let $A$ denote the set of all the sequences of positive integers each of which ends in an infinite string of $1$s. For any two elements $x, y \in A$, let's define $x < y$ if there is some positive ...
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What's so special about the grounded poset of cardinality $2$?

By a grounded poset, I mean a poset $P$ with a bottom element $0_P$. Arrows between grounded posets are monotone mappings that preserve the basepoint. This gives a self-enriched category; lets call it ...
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is an order isomorphism between X and Y also an order embedding from X into Y?

is an order isomorphism between X and Y also an order embedding from X into Y? Please assume that X and Y are both well-ordered sets. I think the answer is yes.. but am not 100% sure. Thanks in ...
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order isomorphism between two intervals, where both have minimum and maximum elements

Note to viewers of this question: My apologies for the late night typos. The answers that I got from James et.al. were great, but targeted to definitions that were in error due to my typos. So those ...
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Finding subgraph in DAG in which all nodes are smaller than all others

I think my problem might be one of terminology, so let me explain what I am trying to do. Given a direct acyclic graph $G$ interpreted as a partial order, I want to find a subgraph $S$, so that ...
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Well ordering agreeing with ordinal ordering on a cardinal

This is an exercise from Kunen book - Set theory, an introduction to independence proofs. Let $\kappa$ be an infinite cardinal and $\triangleleft$ any well-ordering of $\kappa$. Show that there is an ...
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Assume n is an even integer. For an odd integer m, a sequence of m sets S1,…,Sm ⊆ [n] is a graceful chain of length m if…

Assume $n$ is an even integer. For an odd integer $m$, a sequence of $m$ sets $S_1, \dots, S_m \subseteq [n]$ is a graceful chain of length $m$ if: $S_1 \subset S_2 \subset \dots \subset S_m$ For ...
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order isomorphism maps $f < g$ to $Tf < Tg$?

Suppose $C(X)$ is the set of functions $f:X \rightarrow \mathbb{R}$. A bijection $T:C(X) \rightarrow C(Y)$ is called order isomorphism if $f \leq g$ if and only if $Tf \leq Tg$. $f \leq g$ means ...
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Order isomorphism between $(-\infty,0)$ to $(0, \infty)$

Is there an explicit order isomorphic function between $(-\infty,0)$ to $(0, \infty)$? There should be one, because both are order isomorphic to $\mathbb{R}$, but I can't think of one.
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Generalizing a theorem about filters on a boolean lattice

Let $\mathfrak{A}$ be a bounded distributive lattice with binary meet and join $\sqcap$ and $\sqcup$. I will denote $\partial F = \{ X\in\mathfrak{A} \mid \forall Y\in F: X\sqcap Y\ne \bot \}$ where ...
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Uniqueness of completion of dense totally ordered sets

Let $(P, <)$ be a dense totally ordered set. Then there exists a complete totally ordered set $(C, \prec)$, such that: $P \subseteq C$. If $p,q \in P$ then $p<q \iff p \prec q$. $P$ is dense ...
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Is every compact totally ordered space homeomorphic to a subset of $[0,1]$?

Let $(X,\leq)$ be a totally ordered set such that, equipped with the order topology, $X$ is compact. Is then $X$ homeomorphic to a closed subset $A \subseteq [0,1]$? A way to ask this question ...
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An alternate definition of ideals

A filter on a poset if by definition its nonempty subset $F$ such that it does not contain the least element and $A, B \in F \Leftrightarrow \exists Z \in F : (Z \le A \wedge Z\le B)$ for every ...
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Trichotomy implies totality of partial order

Theorem: A partially ordered set is totally ordered if it obeys the law of trichotomy. Things I know: A relation on some set $A$ is said to be a partially ordered set if the relation is reflexive, ...
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Intersection of two filters on a poset

Fix a poset. A filter on the poset is its nonempty subset which is both a down-directed set and an upper set. Conjecture Intersection of two filters is also a filter. I have proved this conjecture ...
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Regarding chains and antichains in a partially ordered infinite space [duplicate]

I've been given this as an exercise. If P is a partially ordered infinite space, there exists an infinite subset S of P that is either chain or antichain. This exercise was given in the Axiom ...
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unclear why subset of a lexicographic ordered set of strings is said NOT to have a least element.

I am watching an excellent series on Discrete Mathematical Structures from IIT At the 47:33 mark of the video the instructor constructs a set of strings as ...
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Generalization of metric spaces?

Usually we define a metric on a space $X$ to be a map $X\times X\to\mathbb{R}$ that satisfies a few axioms. $\mathbb{R}$ has of course a total order. What if we instead have a metric $X\times X\to A$ ...
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Prob. 5, Sec. 4.1 in Kreyszig's functional analysis book: A finite partially ordered set has a maximal element

Let $M \neq \emptyset$ be a finite partially ordered set. Then how to show that $M$ has at least one maximal element? My effort: If $M$ has only one element, then that element is (vacuously) the ...
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When is an ordered space scattered?

There is a concept of scattered in both order theory and topology. A topological space $X$ is scattered if every nonempty subspace has an isolated point. A linearly ordered set $( X , < )$ is ...
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Does $\mathcal{P}(\alpha)$ have maximal well-founded subsets?

Let $\alpha$ denote an ordinal. Then clearly, the powerset $\mathcal{P}(\alpha)$ has well-founded subchains that are maximal with respect to this condition (of being well-founded subchains), since ...
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Proving minimum implies minimal

$$q\in T \text{ is a minimum element of }T\text{ means }(\forall x\in T\setminus\{q\}: q R x\\ q\in T \text{ is a minimal element of }T\text{ means }(\forall y\in T\setminus\{q\}): y !R q.$$ Those ...
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Are all locally finite total orders isomorphic to a subset of Z?

This seems to be an obvious fact. Does anybody know a printed reference? (A partial order is locally finite if all closed intervals are finite.)
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Choosing a $x \in$ $X$, with $X$ an infinte set, $<$ a well-ordering on $X$, such that $x < x'$ for only finitly many $x' \in X$

Let $X$ be an (countable or not) infinte set, $<$ a well-ordering on this set. Lately I read a proof in which was explicity stated to choose an element $x \in X$ such that there are infintly many ...
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Def. 4.1-1 in Kreyszig's functional analysis book: Any example of a subset of a poset with more than one upper bound?

Can we given an example of an element $m \in M$, where $M \neq \emptyset$ is a partially ordered set, such that $m$ is a maximal element of $M$, but $m$ is not an upper bound of any subset $W (\neq ...
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Upper Bounds Of Sub Sets

Prove/Disprove: Let $A\subseteq B\subseteq \mathbb{R}$ If $B$ is bounded from below so $A$ is bounded from below. $B$ is bounded from below $\rightarrow$ $x\leq B$. Let there be $z\in A$, ...
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Having trouble in understanding what subset of a poset means.

I am having a problem to visualise a subset of a poset. Let $S = \{a,b,c\}$. Then $\mathcal{P}(S) = \{\{\},\{a\}, \{b\},\{c\},\{a,b\},\{a,c\},\{b,c\},\{a,b,c\} \}$. Now if I impose $\subseteq$ on ...
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Is there a real number which comes just after a particular real number?

This is like when we say that the integer which comes just after $2$ is $3$. Is there a real number which comes just after a particular real number? For example: Is there a number which comes just ...
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What is known about this well-ordering of the rationals in a finite interval?

Given any interval $I=(a,b) \subset \mathbb R^+$, we may order the rationals in $I$ with a denominator-first lexicographic order, as follows: First, we list, in increasing order of numerator, all $q ...
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minimal embeddings of topological spaces into connected spaces

Defintions: Let $X$ be a topological space. 1) A connected space $Y$ is a minimal connected ambient (m.c.a for short) space for $X$ if there exists an embedding $i:X\mapsto Y$, and for every ...
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A Chain of Subsets of $\mathbb{R}$ Without any Good Countable Subchain

Consider which $\bigl{(} A_i \bigr{)}_{i\in I}$ is a chain of subsets of $\mathbb{R}$. We say that a countable chain like $\bigl{(} B_n \bigr{)}_{n\in \mathbb{N}}$ is good if : for every $n\in ...
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Are these partial order or total orders?

I just want to know if the following are considered partially ordered sets or total set. Here is the definition: $\mathbb{R}^2$ : (a,b) R (c,d) iff $a\leqslant c$ and $b\leqslant d$ $\mathbb{R}^2$ ...
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Relationship between $S$ and $S^{-1}$

This is one of the problem I have been solving in Velleman's How to Prove book: Suppose $R$ is a relation on $A$, and let $S$ be the transitive closure of $R$. Prove that if $R$ is symmetric, ...
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Lexicographic order, $\mathbb{Z^+ \times Z^+}$ $\space \color{red}{ VS} \space $ $\mathbb{Z}$

Why $\mathbb{Z}^+\times \mathbb{Z}^+$ with $(a1,a2) \preceq (b1,b2)$ if $a_1 < b_1$ or if $a_1 = b_1$ and $a_2 \leq b_2$ is a well-order set but $\mathbb{Z}$ set is not a well-ordered one? I read ...
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Does total order imply linearisation

Suppose $X$ is a totally ordered set. Does this mean that $X$ can always be linearised? I mean can $X$ be always written in a linear order like $\mathbb{R}$ ? I came across this question when I was ...
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Characterizing affine subspaces order-theoretically

Let $V$ denote a real vectorspace and $\mathrm{Con}(V)$ denote the poset of convex subsets of $V$. The goal is to identify those elements of $\mathrm{Con}(V)$ that happen to be affine subspaces of $V$ ...
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What do we call well-founded posets whose elements have a unique height?

What do we call those well-founded posets $P$ with the property that for every $x \in P$, all maximal chains in the lowerset generated by $x$ have the same length? Examples: The set of all finite ...
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Zorn's lemma converse? (Context: Maximal proper subgroups)

So, in my qual prep class a pretty simple question popped up: "Prove that for any nontrivial finite group there exists a maximal proper subgroup." So of course, my natural inclination was to ...
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Order by inclusion

Given the set $E = \{a,b,c\}$, order the power set of $E$ (all subsets) by inclusion. I think the order would be $\varnothing$, $\{a\}$, $\{b\}$, $\{c\}$, $\{a,b\}$, $\{a,c\}$, $\{b,c\}$, ...
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$|\alpha\times\alpha|=|\alpha|$ for infinite ordinal $\alpha$, definably

I am trying to prove Proposition 1.7 of http://math.bu.edu/people/aki/7.pdf: Proposition 1.7 (Halbeisen–Shelah). If $\aleph_0\le|X|$, then $|{\cal P}(X)|\not\le|{\rm Seq}(X)|$. In words, ...
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Using substitution to make an equation into a separable differentiable equation

I have the question: By making the substitution $y = t^nz$ and making a cunning choice of n, show that the following equations can be reduced to separable equations and solve them. $$\dfrac{dy}{dt} = ...
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Proving that every submodel of DLOE is an elementary submodel

Let $A$ and $B$ be models of the theory of Dense Linear Orders without Endpoints such that $|B| \subset |A|$. I'm trying to prove that $B$ is an elementary submodel of $A$. Using Tarski-Vaught test I ...
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Does multiplication by the inverse of a Cholesky matrix preserve order?

Let $n \in \{1, 2, \dots\}$ and let $C \in \mathbb{R}_{n, n}$ be a real, symmetric and positive definite $n \times n$ matrix. Define $B \in \mathbb{R}_{n, n}$ to be the real, lower triangular matrix ...