This tag is for set theory topics typically studied at the advanced undergraduate or graduate level. These include cofinality, axioms of ZFC, axiom of choice, forcing, set-theoretic independence, large cardinals, models of set theory, ultrafilters, ultrapowers, constructible universe, inner model ...

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3
votes
1answer
62 views

Are there non-equivalent cardinal arithmetics?

‎Generalizing a concept in mathematics is always a problematic situation. In most cases there are several ways to generalize a notion and it is not easy to decide if a particular generalization is ...
0
votes
1answer
47 views

Topological , Homeomorphic version of $|S \times S|=|S| $

Give example of a subset $A$ of $\mathbb R$ such that with respect to some topology , $ A$ is homeomorphic to $A\times A$ . In set theory ZF it is known to be equivalent to A.C. that for any ...
0
votes
1answer
53 views

understanding cardinal numbers arithmetic

I have a question about notation in a book I'm reading on set theory and beside of my question I will be glad for a recommendation for a good book that explains well cardinal numbers arithmetic. If ...
0
votes
1answer
41 views

continuous poset w.r.t. Scott topology

I am learning continuous poset by myself. I have conclusion as follows: If $P$ is a continuous poset w.r.t. Scott topology then there is $x\in P$ s.t. for any $y\in P$ and for any open sets $U_x$ and ...
34
votes
1answer
396 views

Over ZF, does “every Hilbert space have a basis” imply AC?

I know there is a similar result due to Blass [1] that over ZF, "every vector space has a (Hamel) basis" implies AC. Looking around, however, I can't find any results on the question for Hilbert ...
0
votes
2answers
45 views

Countable collection of countable sets and Axiom of choice

Do we need Axiom of choice(or weaker version axiom of countable choice) to say countable Cartesian product of countable sets is nonempty? I think yes. I read somewhere answer no giving argument: each ...
0
votes
0answers
13 views

Carlson's translatability - are theses characterisations equivalent?

Given a translation-invariant ideal $\mathcal{I}$ on a commutative group $G$ and it's dual filter $\mathcal{I}^*$, I am trying to show that $$ (\forall I \in \mathcal{I})(\exists I' \in ...
1
vote
1answer
60 views

Intuitive diagrams for models of non-well founded set theory

Based on our intuition about von-Neuman's rank, there is a standard view to describe a model of ZFC as a large V-shape world. When we remove the Axiom of Foundation (AF) from ZFC and replace it with ...
4
votes
1answer
51 views

Ordinal exponentiation identity with natural sum of exponents

This is related to a previous question on How to think about ordinal exponentiation? One possible definition for the natural product $\alpha\otimes\beta$ of ordinals is based on Cantor Normal Forms ...
2
votes
1answer
28 views

$\langle \mathfrak{c},\mathfrak{c}\rangle$-Independent Matrix

Given cardinals $\lambda,\kappa$, an $\langle \lambda,\kappa\rangle$-independent matrix on $X$ is a colection $\mathcal{A} = \{A_{\alpha}^{\beta}:\alpha<\lambda\wedge \beta<\kappa\}$ sattisfying ...
6
votes
2answers
67 views

Embedding $\omega_1$ in the hyperreals

I've been trying to show that there exists a subset of the hyperreals which has order type $\omega_1$, that is, that there exists a subset $A \subseteq {}^*\mathbb{R}$ for which there is an order ...
2
votes
1answer
40 views

Constructing almost disjoint families

Let $\mathcal A$ be an almost disjoint family of subsets of $\omega$ and let $\Psi (\mathcal A)$ be the Mrówka space (definition here). Let $$\mathcal I (\mathcal A)=\{X\subseteq \omega : X\subseteq ...
4
votes
1answer
75 views

Proving existence of $\overline{\Bbb Q_p}$ without AC

The proof that every field has an algebraic closure is known to require at least a weak form of AC, the boolean prime ideal theorem. But I recall reading somewhere that for concrete, sufficiently ...
0
votes
0answers
31 views

Least ordinal $\alpha$ that $\omega^\alpha > \beta$ is a successor ordinal

Let $\beta$ be an ordinal. The least ordinal $\alpha$ that $\omega^\alpha > \beta$ is a successor ordinal. This is true, but I'm not sure why. Can someone give me a hint?
7
votes
2answers
235 views

New Axioms of Infinity

Axiom of Infinity says there is an inductive set (i.e. a set which includes $\emptyset$ and is closed under successor operator). Formally: $Inf:\exists x~(\emptyset\in x~\wedge~\forall y\in ...
1
vote
1answer
83 views

Is the set of all cardinals smaller then a strongly inaccessible cardinal closed?

Given a strongly inaccessible cardinal $k$ (i.e. $k$ is regular uncountable and for each $\lambda < k$, $2^\lambda < k$), is the set of all cardinals smaller then $k$ closed or open? Mahlo ...
9
votes
6answers
611 views

Why isn't this a valid argument to the “proof” of the Axiom of Countable Choice?

I am having a little trouble identifying the problem with this argument: Let $\{A_1, A_2, \ldots, A_n, \ldots\}$ be a sequence of sets. Let $X:= \{n \in \mathbb{N} : $ there is an element of the set ...
7
votes
5answers
364 views

When can ZFC be said to be “born”?

The "History" section of the Wikipedia article on ZFC isn't particularly helpful. The only thing I understood from it is that ZFC appeared after 1922. In what book or paper was ZFC first explicitly ...
0
votes
1answer
66 views

How to deal with probability problems in proper class sample spaces?

Here my focus is mainly on $Ord$ and questions such as: An ordinal is chosen by random. What is the probability of the event that it is a cardinal number? A coin is tossed proper class many ...
0
votes
0answers
47 views

Can we unify every pair of inner models of ZFC by a same hierarchy?

Definition: Fix a ground model $V$ of ZFC. Let $F:V\rightarrow V$ be a definable class function (we call it an operator). The hierarchy $W^F$ corresponding to $F$ is defined as follows: ...
12
votes
1answer
222 views

Infinite prisoners with hats — is choice really needed?

The problem is this (recently asked about here): A countably infinite number of prisoners, each with an unknown and randomly assigned red or blue hat line up single file line. Each prisoner faces ...
4
votes
0answers
53 views

For cardinals, if $\mathfrak{a}\ne\mathfrak{b}$ then $2^\mathfrak{a}\ne 2^\mathfrak{b}$

In the usual ZF (or ZFC) set theory, let $\mathfrak{a}$ and $\mathfrak{b}$ be cardinal numbers. Is it correct that one can neither prove nor disprove the statement: $$\mathfrak{a}\ne\mathfrak{b} ...
11
votes
2answers
211 views

What does it mean to say that a particular mathematical theory is a foundation for mathematics?

We usually hear that set theory is a foundation for contemporary mathematics. Category theory is also another foundation of maths. There are other theories which deemed to be a foundation for maths. ...
2
votes
1answer
33 views

Well founded relations.

I'm reading a proof in Jech Set theory and I cannot understand a line. Why is it the case that the replacement axiom guarantees the existence of $\theta$ such that $P_\theta = P_{\theta + 1}$? Last ...
-1
votes
1answer
31 views

A Question Regarding Representing $\mathscr P$($\omega$) as a Digraph and CH

It is well known that one can represent sets as digraphs. What is the proper digraph representation of $\mathscr P$($\omega$)? I ask this because $\mathscr P$($\omega$) is $\Pi_1$ in the Levy ...
0
votes
2answers
70 views

Adding Substructures by Forcing

Consider a first order language $\mathcal{L}$ and an $\mathcal{L}$-structure $M$. Let $V$ be a model of ZFC (or ZF) the general question is that what would happen to classes, $Sub(M):=\{N~;~N~\text{is ...
1
vote
1answer
41 views

invariant well-ordering on binary words

Let $X$ be the set of all infinite to the right binary words, $X^0$ -- the monoid of all finite binary words. There is a natural action of $X^0$ on $X$, namely the concatenation. Does there exists an ...
1
vote
1answer
39 views

Equivalence Relation on a Class

I'm trying to understand Scott's proof of the incompatibility of axiom of constructibility and the existence of a measurable cardinal. I'm stuck in the use of Łoś's Theorem in the universe. Jech's ...
5
votes
1answer
69 views

If $\mathbb{P}$ is a separative poset that doesn't add $\theta$-sequences then every intersection of $\theta$ dense open sets is dense in $\mathbb{P}$

I am looking for a hint (not a solution) to exercise IV.7.28 of Kunen's Set Theory book (2013). Recall that a poset $\mathbb{P}$ is separative if for every $p,q\in \mathbb{P}$, $p\nleq q$ implies ...
1
vote
1answer
27 views

Does a stationary set of a regular cardinal $k$ always contain a segment $(\alpha,k)$ for some $\alpha<k$

As a continuation to this question. Suppose that $A$ is a stationary set of a regular ordinal $k=[0,k)$. Does this equivalent to saying that $A$ contains a subset of the form $(\alpha,k)$ for some ...
3
votes
3answers
75 views

Axiom of infinity and empty set

The axiom of infinity is formulated as $$\exists S ( \varnothing \in S \wedge (\forall x \in S) x \cup \{x\} \in S)$$ Can someone explain why the use of $\varnothing$ in the axiom of infinity makes ...
4
votes
1answer
93 views

How to define $A\uparrow B$ with a universal property as well as $A\oplus B$, $A\times B$, $A^B$ in category theory?

In category theory there are definitions for $A\oplus B$, $A\times B$ and $A^B$ via universal properties. I wonder if it is possible to isolate a particular universal property to represent the ...
1
vote
1answer
47 views

Fixed Point Equivalences of Axiom of Choice

Axiom of Choice has many known equivalences. Also there are many known fixed point theorems (unproved statements) which provide useful information about existence of fixed points for particular ...
34
votes
7answers
2k views

Why is the Continuum Hypothesis (not) true?

I'm making my way through Thomas W Hungerfords's seminal text "Abstract Algebra 2nd Edition w/ Sets, Logics and Categories" where he makes the statement that the Continuum Hypothesis (There does not ...
2
votes
2answers
191 views

Unprovable things

So I was thinking about the statement, "This statement is false". Now I have heard people say that the truth of this is unprovable. But I would say that it is just nonsensical! I.e that it creates in ...
2
votes
0answers
36 views

Is the following notion equivalent to subtle cardinals?

Let $\kappa$ be a regular, uncountable cardinal. We call $\kappa$ $\dagger$ if for every sequence $(A_\alpha \colon \alpha < \kappa)$, $A_\alpha \subseteq \alpha$ and every $\xi_0 < \kappa$ ...
1
vote
0answers
32 views

Looking for example of particular type of proper subgroups of $\big(\mathcal P(A), \Delta \big)$ [closed]

i) Let $A$ be a set with more than two elements , give example of a proper subgroup $H$ of $\big(\mathcal P(A), \Delta \big)$ such that $H$ has more than two elements ii) Let $A$ be an infinite set ...
0
votes
1answer
61 views

How much conservative ZF+AC and ZF+DC are over ZF?

A logical theory $T_2$ is a (proof theoretic) conservative extension of a theory $T_1$ if the language of $T_2$ extends the language of $T_1$; every theorem of $T_1$ is a theorem of $T_2$; and any ...
2
votes
1answer
55 views

Zermelo–Fraenkel Set Theory

So I'll try keeping this real short and simple. Assume that language $L$ is defined as $\{ x\in \{0,1\}^* \}$ (finite binary strings) such that $x$ encodes a proof in ZFC that 4 is a prime number. I ...
3
votes
3answers
159 views

The Free Set Lemma

The statement of the lemma is as follows: if $$f: \omega_1 \rightarrow \{x\ :\ x\ \textrm{is finite}\}$$ then there is an uncountable $S \subseteq \omega_1$ such that for all distinct $\alpha,\ \beta ...
3
votes
1answer
68 views

Suslin Tree implies a Suslin Line

A Suslin line is defined as a non-empty, complete, dense, linear order without endpoints and in which every collection of disjoint intervals is countable. A Suslin tree on the other hand, is an ...
3
votes
1answer
31 views

Should a stationary set of an ordinal contain all it's limit poins?

Should a stationary set of an ordinal contain all it's limit poins? By a stationary set I refer to Jech's definition: If $k$ is a cardinal then a set $S \subset K$ is called stationary if $S \cap C ...
4
votes
1answer
62 views

Proving $V=V_{\sf On}$ in $\sf Z+Reg$

Define the function $$V_0=\emptyset,\qquad V_{\alpha+1}={\cal P}(V_\alpha),\qquad V_{\delta}=\bigcup_{\beta<\delta}V_\beta.$$ Since we are working in $\sf Z$ (i.e. $\sf ZF$ without the axiom of ...
18
votes
6answers
2k views

Foundation for analysis without axiom of choice?

Let's say I consider the Banach–Tarski paradox unacceptable, meaning that I would rather do all my mathematics without using the axiom of choice. As my foundation, I would presumably have to use ZF, ...
2
votes
2answers
59 views

A forcing that is $\omega_1$-closed and $\omega_2$-c.c.

I am reading an article (on second order characterizability) which at some point in a proof states that by forcing with $\mathbb P=\{f:\alpha\to\{0,1\},\alpha\in\omega_1\}$ we do not add subsets to ...
17
votes
1answer
344 views

Why was set theory inadequate as a foundation to the emerging new fields and why category theory isn't?

I'm reading this Wikipedia article on the foundations of mathematics, in the end of the this topic, there is: Toward the middle of the 20th century it turned out that set theory (ZFC or otherwise) ...
2
votes
2answers
114 views

Recursion on a class instead of a set

According to Wikipedia, the recursion theorem states the following: Let $X$ be a set, and $f:X\to X$ a function. For any $x\in X$, there exists a unique function $g:\omega\to X$ such that $g(0)=x$ ...
2
votes
1answer
27 views

to show that there is no injection from a finite successor of finite ordinal to itself

im trying to prove this proposition let $\alpha \in \omega$ so i want to show that there is no injection (not necessarily keeps order) from $\alpha +1$ to $\alpha$ without using cardinality or the ...
3
votes
0answers
104 views

Which infinity do we mean by $\infty$ in the symbol $x\rightarrow\infty$? [closed]

In ordinary mathematics we use the "limits" frequently. In principle the notion of "limit" is closely related to the notion of "infinity". Intuitively when we are calculating a "limit" we begin from ...
1
vote
0answers
46 views

Is there any inconsistent large cardinal axiom which its inconsistency proof is essentially different from proof of Kunen inconsisteny theorem?

There is a long list of large cardinal axioms. Most of them deemed to be consistent with ZFC but there are also some axioms like existence of Reinhardt or $\omega$-huge cardinals which are natural ...