All Questions
Tagged with set-theory foundations
157 questions
6
votes
2
answers
376
views
Resource request on "$\in$-homomorphisms" in Set Theory
Very loosely put, this is the intuitive idea behind an $\in$-homomorphism:
Let $\mathcal{U}$ and $\mathcal{W}$ be universes of sets. A function $f \colon \mathcal{U} \to \mathcal{W}$ is said to be an $...
- 61
10
votes
1
answer
451
views
Is material set theory conservative over structural set theory?
Suppose a statement $\phi$ that doesn't use the global $\in$-relation or the global $=$-relation in an essential way is provable in some material set theory, say bounded Zermelo with choice. (So that ...
- 101
17
votes
2
answers
2k
views
When the definition of a set starts to matter in category theory
In most introductory courses to category theory, the precise definition of a set is more-or-less ignored. The idea being that all basic results in the subject hold for any reasonable definition of a ...
- 141
157
votes
5
answers
28k
views
What makes dependent type theory more suitable than set theory for proof assistants?
In his talk, The Future of Mathematics, Dr. Kevin Buzzard states that Lean is the only existing proof assistant suitable for formalizing all of math. In the Q&A part of the talk (at 1:00:00) he ...
- 1,667
7
votes
1
answer
311
views
What is difference between working with small and large category of spaces?
The following construction have always bugged me. This is p328, Remark 5.1.6.1 in Lurie's Higher Topos Theory. Lurie begins with the following:
Construction: Let $C$ be a simplicial set. $S$ denote ...
- 661
8
votes
4
answers
775
views
Self-contained formalization of random variables?
I have not been able to find any formalization of random variables that supports construction of new random variables dependent on previously constructed ones. In what I have found, a random variable $...
- 2,912
6
votes
1
answer
309
views
Set Theoretic Geology II: The structure of the directed partial order of grounds
In my previous question Set-theoretic geology: controlled erosion?
and the great answer by Jonas Reitz, I have learned a few things, starting from the awareness that I understand the fine-grain ...
- 7,853
5
votes
0
answers
278
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Class theory of ZF-minus-Powerset as classical predicative system?
I've been thinking about some mathematics in weaker foundational systems a little bit, largely from a structural viewpoint, and with particular attention to classes.
Some categories I've been keeping ...
- 35.5k
1
vote
1
answer
396
views
Complete and consistent first-order theories that contain interesting phenomena
Gödel has shown that a consistent recursively axiomatizable first-order theory that can interpret Robinson arithmetic is incomplete.
I think there is some sentimental value in working with a theory ...
user158636
63
votes
4
answers
7k
views
When size matters in category theory for the working mathematician
I think a related question might be this (Set-Theoretic Issues/Categories).
There are many ways in which you can avoid set theoretical paradoxes in dealing with category theory (see for instance ...
- 3,318
6
votes
1
answer
545
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Historical origin of the empty set
The question is in the title:
Who first claimed the existence / necessity of the empty set ? When did this happen ?
Of course I know that the notation $\emptyset$ goes back to André Weil, and that ...
- 52.3k
74
votes
8
answers
14k
views
Category theory and set theory: just a different language, or different foundation of mathematics?
This is a question to research mathematicians, as well as to those concerned with the history and philosophy of mathematics.
I am asking for a reference. In order to make the reference request as ...
- 6,917
19
votes
1
answer
937
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Positive set theory and the "co-Russell" set
This is a more focused version of a question which was asked at MSE a couple years ago, but is still unanswered there. That question asks about a broad range of theories, whereas this version focuses ...
- 21.2k
3
votes
2
answers
719
views
Shortest axiom of infinity for foundationless set theory
Let $T$ be the theory with a binary symbol $\in$, an unary symbol $S$, and the following axioms:
Axiom of extension:
\begin{equation}
\forall x \forall y (\forall z (z \in x \leftrightarrow z \in ...
- 2,203
6
votes
3
answers
2k
views
How strong is this set theory?
In the spirit of this related question, consider a set theory with the following axioms:
Axiom of extension:
$$ \forall x \forall y (\forall z (z \in x \leftrightarrow z \in y) \rightarrow x = y) $$
...
- 2,203
16
votes
2
answers
2k
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Could Kronecker accept a proof of Goodstein's theorem?
A famous result of Goodstein asserts that the Goodstein sequence of integers terminates.
For a precise statement and a short proof, see https://en.wikipedia.org/wiki/Goodstein%27s_theorem.
A well ...
- 28k
38
votes
4
answers
6k
views
Could groups be used instead of sets as a foundation of mathematics?
Sets are the only fundamental objects in the theory $\sf ZFC$. But we can use $\sf ZFC$ as a foundation for all of mathematics by encoding the various other objects we care about in terms of sets. The ...
- 3,137
2
votes
1
answer
458
views
Set of definable real numbers?
Is there a set theory at least as strong as $KP\omega$ which has as a theorem that there is a set $\mathbb{D}$ of precisely the definable real numbers?
- 3,574
3
votes
0
answers
301
views
What does second order set theory give us that is new?
There is a natural analogy between the theories PA and ZFC. See the linked question by Gro-Tsen here.
Peano arithmetic (PA) is a first order approximation to the natural numbers. As is well known, ...
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-1
votes
1
answer
291
views
Weak power set - what strength may it have? [closed]
In The Consistency of Classical Set Theory Relative to a Set Theory with Intuitionistic Logic in THE JOURNAL OF SYMBOLIC LOGIC Volume 38, Number 2, June 1973 page 316 Harvey Friedman's axiom 8* $Weak \...
- 3,574
1
vote
0
answers
278
views
A countable set theory providing choice?
Instead of Zermelo set theory $Z$ take $Y$ = $Z$ minus the power set axiom plus
Enumerability: $\forall x(x\neq \emptyset \to\exists f[f:\mathbb{N}\overset{onto}{\frown}x ])$
$\imath$ is the ...
- 3,574
5
votes
1
answer
492
views
Is ETCS well-founded?
I can't find a statement about the axiom of regularity anywhere in treatments of ETCS. Perhaps this is due to the unfortunate clash of terminology with 'foundations'.
- 1,083
6
votes
1
answer
937
views
Smallest ordinal modelling $\aleph_1$?
Let $X_1$ be the class of all ordinals $\alpha$ such that there exists a transitive model $M$ of ZF(C) such that $M$ thinks that $\alpha$ is $\aleph_1$.
Every class of ordinals has a minimum element (...
- 395
4
votes
0
answers
140
views
Is well-pointedness the reason that the internal/external distinction seems not to apply to $\mathbf{Set}$?
When reasoning about the category of sets, we usually don't have to worry about the internal/external distinction. For example, if $f : X \rightarrow Y$ is a morphism of sets, then $f$ is either ...
- 3,793
0
votes
1
answer
295
views
Formalizing ontological optimism
Inform speaking ontological optimisms means that everything that possibly exists in the abstract reality actually exists. From this principle we (again informally) get the Axiom of infinity, the Power ...
- 1,648
2
votes
0
answers
159
views
Why not replace reflection by bounded reflection in Muller's approach?
Bounded Reflection: If $\phi$ is a formula in the language of set theory [i.e.; $\small \sf FOL(=,\in)$], in which all and only symbols $``x,x_1,..,x_n"$ occur free, and $\phi^V$ is the formula ...
- 11.3k
1
vote
2
answers
228
views
Cardinals in $ZFC+\neg CH$
Before asking my question I like to admit that i am not an expert in the foundation of mathematics and I am interested in this issue more from a philosophical perspective. So my question may be ...
- 1,648
6
votes
1
answer
994
views
Which branches of mathematics can be done just in terms of morphisms and composition?
Consider the first-order language $L_{\omega\omega}$ of the signature $L:=\{\mathrm{dom}, \mathrm{cod}, \mathrm{comp}\}$, where $\mathrm{dom}$ and $\mathrm{cod}$ are unary function symbols and $\...
user103598
7
votes
2
answers
1k
views
Explaining the consistency of PRA and ZF from predicative foundations
Recently I got interested in predicative foundations, mostly because of Laura Crosilla's work and because Agda employs a predicative type theory.
From the point of view of a predicative foundation to ...
- 3,312
9
votes
1
answer
687
views
"Surjective cardinals" - using surjections rather than injections to define isomorphism classes of sets
Cantor used the notion of an "injection" to formalize the size of two sets: A is "smaller" than B if A injects into B.
Simply put, the question is - how does this situation change if we use ...
- 4,897
12
votes
1
answer
1k
views
ZF(C) and category theory
Is there an axiomatisation of some kind of category theory and a definition of sets in this framework such that the axioms of ZF resp. ZFC are theorems?
- 1,648
2
votes
0
answers
305
views
Does this axiomatic system satisfy requirements for founding mathematics?
In this article, the author, F.A.Muller, suggests criteria for a founding theory of mathematics (pp:14-16). The author proposes $ARC$ Class Theory to embody these requirements. The motivation is ...
- 11.3k
3
votes
0
answers
283
views
Formal foundations done properly [closed]
I would want to do mathematics properly, so that the proofs of results can be trusted on instead of them being just suggestions on which results could perhaps apply. This means formulating the math in ...
- 31
6
votes
3
answers
446
views
How do we formally construct the successor universe $\mathscr{U}^+$ of a universe $\mathscr{U}$ in $\mathsf{ZFC}$?
A set $\mathscr{U}$ is a universe if the following conditions are met:
For any $x \in \mathscr{U}$ we have $x \subseteq \mathscr{U}$
For any $x,y \in \mathscr{U}$ we have $\{x,y\} \in \mathscr{U}$,
...
- 1,115
12
votes
1
answer
603
views
Translating Grothendieck axiom UB into ZFC
In SGA 4, Grothendieck introduced a set-theoretic device called a Grothendieck universe. He and his collaborators worked in Bourbaki set theory, which is practically similar to $\mathsf{ZFC}$ but ...
- 1,115
7
votes
1
answer
665
views
Enhancing Grothendieck's universes and Grothendieck's axiom: Feferman's universe
A Grothendieck's universe is such a set $U$ so that
$\forall x \in U, x \subseteq U$,
$\forall x,y \in U, \{x,y\} \in U$,
$\forall x \in U, \mathcal{P}(x) \in U$,
given a family $(X_i)_{i \in I}$ ...
- 1,115
5
votes
3
answers
488
views
Counting without one-to-one correspondence? [closed]
Ash and Gross in their wonderful book Fearless Symmetry found it worth mentioning (and thus suggesting) another way of counting for which "we do not even need to know how to count" (in the sense of ...
- 9,720
16
votes
3
answers
1k
views
Consequences of foundation/regularity in ordinary mathematics (over ZF–AF)?
This is a followup question to Does foundation/regularity have any categorical/structural consequences, in ZF?
As shown in answers to that question, the axiom of foundation (AF, aka regularity) has ...
- 21.3k
10
votes
1
answer
1k
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Erroneous proof of recursion theorem examples
In his book Elements of Set Theory, Herbert Enderton defines (p. 70) a Peano system as a triple $(N, S, e)$ where $N$ is a set, $S$ is an $N$-valued function defined on $N$ and $e$ is a member of $N$ ...
- 209
15
votes
2
answers
1k
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Does foundation/regularity have any categorical/structural consequences, in ZF?
(Prompted by reflection on this old answer, and its suggestion of the “harmlessness” of the axiom of regularity.)
In ZFC, one may justify the axiom of foundation (AF, aka the axiom of regularity) as ...
- 21.3k
4
votes
0
answers
424
views
What are the requirements of a foundational theory?
There are multiple languages to describe all of mathematics, and there are some equivalences between them, some more successful then others.
My question is can we describe some requirements (in some ...
- 773
-2
votes
1
answer
317
views
Is it natural to hold that Ur-elements, small & big sets and proper classes exists? [closed]
The topic of this post was shifted to
https://philosophy.stackexchange.com/questions/49504/is-it-natural-to-hold-that-big-sets-and-proper-classes-exist
Since it was deemed to be a philosophical ...
- 11.3k
39
votes
7
answers
6k
views
Is V, the Universe of Sets, a fixed object?
When I first read Set Theory by Jech, I came under the impression that the Universe of Sets, $V$ was a fixed, well defined object like $\pi$ or the Klein four group. However as I have read on, I am ...
- 750
2
votes
0
answers
92
views
Constructing the Von Neuman Hierarchy at ω+ω in a structural set theory
I'm working in SEAR which is a relatively new structural set theory, and I am trying to prove the existence of big sets.
SEAR has the collection axiom which is, loosely speaking, that for every ...
8
votes
1
answer
1k
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Category theory without axiom of choice
I'm looking for references on the development of (some of) Category theory without the axiom of choice. One possible axiom system (that, to me, seems the natural setting) is ZF + there are arbitrarily ...
- 773
19
votes
3
answers
1k
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Large categories vs. $\mathrm{U}$-categories: why is the loss of category-theoretic information inessential?
I've asked a related question about nine months ago here, however, apparently, I lacked expertise to ask the precise question I want to ask here, as I wish to revisit the matter of universes. I hope ...
- 1,115
4
votes
2
answers
548
views
Anti-foundational set theory with a universal set
There are alternative set theories that allow for a universal set, e.g. NF(U), positive set theory and and topological set theory. There are also alternative set theories like ZFA that allow for the ...
- 1,228
8
votes
1
answer
1k
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Does equality between sets contradict the philosophy behind structural set theory?
Zermelo-Fraenkel set theory (with choice) is commonly accepted as the standard foundation of mathematics. It is a material set theory. This means that for every two objects/sets $a,b$ one can ask ...
- 89
2
votes
0
answers
150
views
Is there equality between sets in structural set theory?
In material set theory, the axiom of extensionality defines equality between sets: two sets are equal iff they have the same elements. In structural set theory, one cannot formulate this.
But however,...
- 21
9
votes
1
answer
1k
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How are material set theory and structural set theory related from the point of view of category theory?
In his comments to both cody and Nik Weaver regarding his answer to user7280899's mathoverflow question "What kind of foundation are mathematicians using when proving metatheorems?", Mike Shulman ...
- 6,132