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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Inner Models of ZFC which satisfy V=L [on hold]

Let $M$ be an inner model of ZFC which satisfies the Axiom of Constructibilty ($V=L$). What is known about general form of such a model? Should it be similar to Godel's constructible universe ($L$) in ...
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28 views

Is it possible to iterate a function transfinite times?

Let $f:A\rightarrow A$ be a function. We can simply define $f\circ f$, $f\circ f\circ f$, etc., for each given natural number inductively. $f^{(0)}=id_A$ $\forall n\in\omega \qquad f^{(n+1)}=f\circ ...
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1answer
37 views

Miller's Construction, Partition Principle and Failure of Axiom of Choice

Partition Principle ($PP$) is the following statement: For all sets $a$, $b$ there is an injection $f:a\rightarrow b$ iff there is a surjection $g:b\rightarrow a$ It is known that $ZF\vdash ...
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33 views

Is this an equivalent form of Axiom of Choice? [duplicate]

It is known that Axiom of Choice implies the following statement: For each two sets $A$ and $B$, there is a one to one function from $A$ to $B$ iff there is a function from $B$ onto $A$ Is above ...
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2answers
147 views

How many undecidable statements are there in ZFC?

There are several statements known to be undecidable in ZFC, with the continuum hypothesis probably being the most "popular" one. Is it known how many undecidable statements are there in ZFC? I.e. ...
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1answer
79 views

Why is the powerset axiom more acceptable than the axiom of choice?

The key step in Zermelo's proof of the well ordering theorem is to use $\text{AC}$ to simultaneously choose the next elelment for all possible partial chains in prospective well orderings, but that ...
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64 views

Does such a first-order theory exist? A question pertaining to transitive models of ZFC.

Assume a proper class of inaccessibles. Does there exist a first-order theory $T$ subject to the following conditions? $T$ admits an infinite model Whenever $M$ is a transitive model of ZFC with $T ...
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1answer
148 views

Is there “intuition” as to why the Continuum Hypothesis is independent of most large cardinal axioms?

I could not find a question that seemed to answer my specific query, despite lots of material on the Continuum Hypothesis (CH) on MSE and MO. If there is already a question on this, I'd greatly ...
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58 views

how many infinities are there? [duplicate]

I'm a past-graduate in mathematics and familiar with the basics of ordinals and cardinals. My question is: how many infinities are there? There are obviously infinitely many, but since we already know ...
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57 views

Manifolds of Non-Standard Dimension

Can there exists (non-trivial) manifolds of non-standard dimensions? Certainly, there do exist manifolds of dimension $n$ for any $n \in \mathbb{N}$ (as well as manifolds of countably many ...
3
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1answer
42 views

$E$ stationary, $D$ closed and unbounded, then $E \cap D$ stationary.

A subset $S$ of $\omega_1$ is called stationary if the image of every normal function on $\omega_1$ has a non empty intersection with $S$. Let $E$ be a stationary subset of $\omega_1$, and let $S:= ...
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101 views

How much maths can we do in NF(U)?

I have recently become interested in non-standard set theories, particularly in NF and NFU and have been reading some things here and there. Of course, I don't know much about it and I'm still trying ...
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72 views

Understanding the Banach-Tarski Paradox

How is it possible to prove a paradox? Also, can someone explain the Banach-Tarski paradox in layman's terms (for someone up to calc 3 and ODEs knowledge)?
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1answer
119 views

All games determined + ZF inconsistent

Let $A$ be a nonempty set, $T\subset A^\mathbb{N}$ a nonempty pruned tree and $X\subset [T]$. The game $G_{A}(T,X)$ is played as follows: Player I and Player II take turns playing $a_{0},a_{1},\dots$ ...
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1answer
137 views

Can set theory be inverted?

Has anybody investigated or constructed a set theory, call it $T$, which develops in a manner that is the reverse of how the majority of the standard set theories develop? Instead of starting with the ...
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88 views

Absoluteness of $\forall x (x=x)$

Is there any kind of (set-theoretic) absoluteness result for the formula $\forall x (x=x)$? And what about for $\exists x (x=x)$? I know $x=x$ is absolute given that it's $\Delta_0$. Also, if I'm ...
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2answers
77 views

Trouble understanding Jech's version of Easton's theorem

On page 232 of Jech's Set Theory (3rd Edition, 2003), we have the following statement of Easton's theorem. Theorem 15.18 (Easton). Let $M$ be a transitive model of ZFC and assume that the ...
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1answer
29 views

Definition of an $E$-rudimentary function

For a given set or class $E$, we call $f: V^k \rightarrow V$, where $k < \omega$, $E$-rudimentary, iff it can be generated by the following schemata: $f(x_1,\ldots,x_k) = x_i$ $f(x_1,\ldots,x_k) ...
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2answers
115 views

Can a set have a subset which doesn't exist?

Is it possible in ZF that given some set $S$, we can informally "describe" a set $P$ such that $P \subseteq S$, and $P$ does not exist (or we can not prove within ZF that P exists)? In other words is ...
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1answer
50 views

Does the converse of Tychonoff's theorem hinge on the axiom of choice?

Tychonoff's theorem:$\phantom{---}$ If $A$ is a non-empty index set and $X_{\alpha}$ is a non-empty compact topological space for every $\alpha\in A$, then $X\equiv\times_{\alpha\in A} X_{\alpha}$ is ...
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2answers
64 views

Clarifications on proof of compactness theorem

I've been reading through the following proof of compactness theorem: http://www.princeton.edu/~hhalvors/teaching/phi312_s2013/compactness.pdf One thing that struck me is that this proof seems to ...
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1answer
30 views

Range of elementary embedding $\pi: V \rightarrow M$ models ZFC?

Let $V$ denote the cumulative hierachy and $M$ be a class together with an elementary embedding $\pi: V \rightarrow M$. As $\pi$ is elementary, we get that $im(\pi)$ models ZFC. But now my textbook ...
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1answer
40 views

Elementary embeddings and continuity

Let $\pi: V \rightarrow M$ be a non-trivial elementary embedding with critical point $\kappa$, where $M$ is a transitive class. I don't seem to understand a given proof of the following basic ...
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34 views

$\left(V_{\pi(\alpha)} \right)^M = \pi \left( V_\alpha \right) $, where $\pi: V \rightarrow M$ is an elementary embedding

Let $\pi: (V;\epsilon) \rightarrow (M;\epsilon \restriction M)$ be an elementary embedding from $V$ into a transitive class $M$. Furthermore, let $V_0 = \emptyset$. $V_{\alpha+1} = \mathcal ...
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2answers
44 views

Is it possible to prove that $x=\{x\}$ is false in ZF system? [duplicate]

A object is different from the set containing that object seems a basic idea of set theory. That is, for any object $x$, $x≠\{x\}$. But I don't know how to prove it in ZF system (Zermelo-Fraenkel ...
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72 views

$\aleph_\alpha^{\aleph_1} = \aleph_\alpha^{\aleph_0}\cdot 2^{\aleph_1}$ for all $\omega \le \alpha < \omega_1$

I'd like someone to check my proof that $\aleph_\alpha^{\aleph_1} = \aleph_\alpha^{\aleph_0} \cdot 2^{\aleph_1}$ for all $\omega \le \alpha < \omega_1$. First, let's prove that ...
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1answer
43 views

$\{\alpha < \kappa \mid cf(\alpha) = \lambda\}$ is not ineffable

We call a subset $X \subseteq \kappa$ of a regular cardinal $\kappa$ ineffable, iff for every family $(A_\alpha \mid \alpha \in X)$ of subsets $A_\alpha \subseteq \alpha$, there is a stationary set $S ...
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1answer
59 views

Product of Borel $\sigma$-algebras vs Borel $\sigma$-algebra of product

If $X$ and $Y$ are topological spaces with associated Borel $\sigma$-algebras $\mathcal{B}_X$ and $\mathcal{B}_Y$, then the product $\sigma$-algebra $\mathcal{B}_X\otimes \mathcal{B}_Y\subset ...
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1answer
63 views

Question about cardinals in ZF

In Lévy's basic set theory refers a theorem of Bernstein as exercise problem: Theorem. (Bernstein) Let $\mathfrak{a,b}$ be cardinals. If $\mathfrak{a+a=a+b}$, then $\mathfrak{b\le a}$. I try to ...
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2answers
203 views

Why exactly is Whitehead's problem undecidable.

I'm trying to get a deeper understanding of Whitehead's problem. It is possible to construct a group of cardinality $\aleph_1$ that satisfies Chase's condition, and is not free. This group is ...
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48 views

Cardinality of Countable Support Iteration of Proper Forcing

Suppose $\mathsf{CH}$ holds. Suppose $\mathbb{P}$ is a forcing such that $\mathsf{ZFC} \models \mathbb{P} \text{ is proper and } |\mathbb{P}| = 2^{\aleph_0}$. For instance, Sacks forcing is such an ...
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1answer
44 views

Functions operating in uncountable sets with cardinality $\gt\aleph_1$

A generic function $y=f(x)$ maps a number in the set of real number $X$ in another number in the set $Y$. It's well known that the irrational numbers are not countable. It's also known we can get a ...
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1answer
90 views

Proper Forcing and Sequence of Names for Reals.

I read something that seems to suggest the following is true: If $\mathbb{P}$ is a proper forcing, $|\mathbb{P}| = \aleph_1$, and $\mathsf{CH}$ holds, then there exists a sequence $\{(p_\xi, ...
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1answer
66 views

Godel's completeness theorem and formula that states consistency of ZF

Godel's completeness theorem, in original formulation, says that every logically valid statement/formula has finite deduction of a formula. Now then there is Godel's incompleteness theorem. Would this ...
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56 views

Are there axiomatizations of first order logic or set theory defined in first order logic or set theory?

There are several axiomatizations for number theory, group theory, and other theories represented in first order logic. Further, these theories are also representable in set theory such as $\sf ZFC$ ...
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65 views

Is it possible for infinite sets to exist in ZFC with the negation of the Axiom of Infinity? [duplicate]

The Axiom of Infinity states that at least one inductive set exists. Inductive sets are infinite, but not all infinite sets are inductive. Suppose that we take ZFC with the negation of the Axiom of ...
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2answers
87 views

Uncountable models for integers

Part of Asaf Karagila's brilliant answer to one of my other questions puzzles me a lot. Namely, I find it hard to understand how there can be a model for ZFC with uncountably many integers. My ...
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1answer
89 views

Maximal model for $\Bbb R$?

I have not dealt professionally with set theory, so excuse me if my way of formulating this question does not completely follow standard terminology. Actually, my question is about whether or not the ...
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44 views

A problem with an assumption in a previous lemma for the proof of Silver´s Theorem on SCH in Jech´s “Set Theory”

In the Jech´s textbook proof of Silver´s Theorem, specifically in the Lemma 8.15, there are an assumption in the beginning of the proof. First, the Lemma 8.15 says that, under the assumption that ...
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59 views

Can we consider each first order structure as a pure set?

Pure sets are the simplest kind of first order structures in the language $\mathcal{L}=\emptyset$. As same as any other generalization, the notion of a first order structure preserves some properties ...
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50 views

Is consistency strength order dense?

Definition: Let $\sigma$, $\theta$ be two statement in the language of set theory. We say $\sigma <_{c} \theta$ ($\sigma$ is strictly lower than $\theta$ in consistency strength order) if within ...
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1answer
48 views

A question about second-order logic and inaccessible cardinals.

Let $\kappa$ denote an inaccessible cardinal, and suppose $T \in V_\kappa$ is a second-order theory. Now consider some mathematical structure $X \in V_\kappa$. Then I think it is clear that $X \models ...
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1answer
24 views

Writing the ZF formula for the choice function (given Well-ordering)

If every set has a well order, then the axiom of choice follows: Given a well order on $\bigcup_{i \in I} A_i$ we define the choice function in this way $f(i) =$ "the first element of $\bigcup_{i \in ...
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1answer
35 views

axiom of regularity and power sets [closed]

So axiom of regularity works for any non-empty set. Is that means that a set like this {aab, +, 50, ), (, **} and all of it's subsets are actually made of empty sets? And not just sets, but power ...
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1answer
135 views

Why is ZF favoured over NBG

So why is ZF favoured over NBG? Is it historical, but I've read after Gödel's monograph was published, NBG was more prominent. Or is the reason that NBG gets the cold shoulder to do with forcing and ...
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1answer
52 views

Comprehension and Impredicativity

Wang and McNaughton (Les Systemes Axiomatiques de la Theorie des Ensembles, 1953) discuss briefly the topic of impredicativity in chapter 2 (titled 'Type Theory') of the above mentioned book, but I'm ...
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1answer
20 views

Redundancy in definiton for forcing poset

The following definition appears in Kunen (2nd edition): For any sets $I,J$ and cardinal $\lambda$: $\text{Fn}_{\lambda}(I,J)$ is the set of all $p\in{[I\times{J}]}^{<\lambda}$ such that $p$ is ...
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1answer
38 views

What is the definition of the Feferman-Levy model?

Any (reference to) definition of Feferman-Levy model in set theory? I cannot find any... Though I know what is Levy collapse.
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1answer
16 views

Uniqueness in transfinite recursion.

Theorem of transfinite recursion: Given a well-ordered set $A$ let $\varphi(g,y)$ be a ZF formula such that for every $a \in A$ and every function $g$ with domain $I_a$ (where $I_a$ is the initial ...
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1answer
58 views

If a version of GCH holds for Chang's $\kappa$-constructibility, does a version of GCH hold for $L_{\infty}$?

In C.C. Chang's paper "Sets Constructible Using $L_{\kappa \kappa}$" one can "deduce a version of the GCH, theorem VI [(iv)--my comment], assuming that all sets are $\kappa$-constructible." Now ...