# Questions tagged [metamathematics]

the mathematical discipline that applies mathematical methods to the study of mathematical theories themselves.

97
questions

-2
votes

0
answers

102
views

### Math that does not have infinity [migrated]

I am not a mathematician. So I am not even sure if what I am asking is logically coherent. But I do have some application-based curiosity that I would like to enlighten myself about. I will first pose,...

4
votes

0
answers

138
views

### Are the relations of being homeomorphic or being homotopy equivalent on the compact polyhedra definable in the structure of the natural numbers?

Let $ K(\mathbb{N}) $ denote the set of all finite simplicial complexes with vertices in $\mathbb{N}$.
Let $ f\colon \mathbb{N} \to K(\mathbb{N}) $ be a computable bijection.
Let $ R $ = { $ (m, n) $ |...

-5
votes

1
answer

430
views

### How much would a mathematician cost? [closed]

Recently our department lost one of the best professors who was attracted by a better University. If we were a football club, and he were a leading player, we would receive many millions of ...

3
votes

1
answer

135
views

### Does CZF prove there is a minimal cauchy completion of the rationals?

In IZF, we can easily prove there is a minimal cauchy complete field extending the rationals: the dedekind reals are cauchy complete, so just intersect all of its cauchy complete subfields.
CZF can ...

2
votes

0
answers

224
views

### A formal definition of a useful theorem?

Sorry if this feels a bit squishy, but I'm wondering if there is any published work trying to give a fully formal definition of the notion of a useful theorem. I mean, in mathematics we all know that ...

2
votes

0
answers

94
views

### How restrained are we in terms of metatheories when working with higher order logics with full semantics?

When working in the realm of first order logic one can use very basic mathematical backgrounds(in reverse mathematical sense) to prove interesting things about more "structures".
In what ...

7
votes

2
answers

392
views

### Equivalences of $n$-categories

This question is an extension of my previous question last year (see [2020]) in which I asked about the (consensus of a) definition of a weak $n$-category.
Here are some background: while strict $n$-...

33
votes

3
answers

5k
views

### Top-down mathematics, or "Where it all begins"

Sorry if this is off-topic.
It was my attempt to take a top-down approach to mathematics.
Being an inexperienced undergraduate (so please take my writing here lightly), I've been presented with ZFC as ...

12
votes

1
answer

881
views

### So after all, what is this thing about topos theory and non-binary truth?

Disclaimer. The question below is necessarily vague. I understand neither the subject matter topos theory nor the object about which my question is (the construction of a fractional / non-binary / ...

16
votes

3
answers

1k
views

### Is there a semantics for intuitionistic logic that is meta-theoretically "self-hosting"?

One can study the standard semantics of classical propositional logic within classical logic set theory, so we can say that the semantics of classical logic is meta-theoretically "self-hosting&...

3
votes

1
answer

408
views

### Is there an equivalent of the incompleteness theorems/halting problem in category theory?

Taking the doctrine of computational trinitarianism ( https://ncatlab.org/nlab/show/computational+trinitarianism ), if one understands the incompleteness theorems as the "logic" version, and ...

26
votes

3
answers

3k
views

### What sorts of extra axioms might we add to ZFC to compute higher Busy Beaver numbers?

First, some context. Ever since I was a high schooler, I have been fascinated with large numbers. As I have grown in mathematical maturity, I have become both disappointed and fascinated to see that ...

4
votes

0
answers

376
views

### How can I prove that primitive recursion “preserves” representability in Peano Arithmetic?

I'm working on my thesis about Gödel's Incompleteness Theorems, and at some point I need to prove that the $\textsf{PA}$ system is able to represent all the recursive functions.
By recursive function ...

8
votes

1
answer

433
views

### Natural $\Pi_1$ sentence independent of PA

Order invariant graphs and finite incompleteness by Harvey Friedman gives an example of a combinatorial/non-metamathematical $\Pi_1$ sentence that is independent of ZFC. Is there a simpler example of ...

-4
votes

1
answer

192
views

### Is there a procedure to derive models from axiomatic systems? [closed]

Is there a systematic procedure to construct a model of an axiomatic system from the system itself?
For example given the abstract postulates of a ring we can show that the integers satisfies them ...

2
votes

1
answer

263
views

### Possible to prove a lemma from Godel's completeness theorem in intuitionistic logic?

I am trying to determine whether a particular theorem used in the course of Godel’s (1930) proof of the completeness of predicate logic could be proven in an intuitionistic metatheory.
Theorem VI (p....

8
votes

0
answers

640
views

### Is there any theorem achieving Conway's "Mathematician's Liberation Movement"

John Conway on in the appendix to part zero of ONAG describes a "Mathematician's Liberation Movement". The goal would be to give mathematicians the freedom to create mathematical theories with the ...

2
votes

1
answer

387
views

### ZFC ability to express truth and $\omega$ - consistency

Some theories can lie about their own consistency (for example, if $PA$ is consistent then the theory $PA + \lnot CON(PA)$ is consistent, although it proves its own inconsistency).
Now working with ...

1
vote

0
answers

87
views

### Questions in proof theory (PRA-provability of EA-theorems, Girards book from '87)

I've been working through a textbook, often encountering difficulties with the exercises.
On mathstackexchange, with most of them I haven't arrived yet at a satisfactory solution.
As I understand, ...

0
votes

0
answers

157
views

### Are the HBL derivability conditions necessary for Gödel's incompleteness theorems? (For Löb's theorem?)

I've been working through a textbook, often encountering difficulties with the exercises.
On mathstackexchange, with most of them I haven't arrived yet at a satisfactory solution.
As I understand, ...

1
vote

0
answers

106
views

### n-consistency - provability/truth of $\Sigma^0_n$ and $\Pi^0_{n+1}$ -formulas; n-consistent extensions, etc

I've been working through a textbook, often encountering difficulties with the exercises. On mathstackexchange, with most of them I haven't arrived yet at a satisfactory solution.
As I understand, ...

18
votes

1
answer

348
views

### Proof as a Σ₁ approximation to truth: what about higher degrees?

Let us posit that the goal of mathematics is to study mathematical truths, and let us stick to arithmetic for simplicity: so let $\mathscr{T} \subseteq \mathbb{N}$ be the set of (Gödel codes of) true ...

5
votes

4
answers

1k
views

### How can you formalize the metamathematics conventionally used to state Godel’s theorem?

Gödel’s incompleteness theorem states that for any sufficiently strong formal system $T$ there exists a statement $G$ such that if $T$ is consistent, then $G$ is true but not provable in $T$. But I’m ...

3
votes

1
answer

273
views

### Unorthodox constructive reasoning: The Kleene Getaway

KG (the Kleene Getaway) is the name I improvised (in my answer to a question on MO on constructive Perron-Frobenius) for a constructive principle which enables the direct constructive use of a ...

3
votes

0
answers

269
views

### Can we prove the epsilon theorems without the axiom of choice?

Hilbert's epsilon $\epsilon$ is a quantifier. It follows the rule that if $\exists x. p(x)$, for some predicate $p$, we can infer $p(\epsilon x. p(x))$. Semantically, it represents picking some ...

7
votes

2
answers

641
views

### Is every true statement independent of $PA$ equivalent to some consistency statement?

Most true statements independent of PA that I know of is equivalent to some consistency statement. For example
Con(PA), Con(PA + Con(PA)), Con(PA + Con(PA) + Con (PA + Con(PA)), $\dots$
Goodstein's ...

17
votes

1
answer

2k
views

### What is known about the relationship between Fermat's last theorem and Peano Arithmetic?

As far as I know, whether Fermat's Last Theorem is provable in Peano Arithmetic is an open problem. What is known about this problem?
In particular, what is known about the arithmetic systems $PA + \...

4
votes

2
answers

472
views

### Experiments physically performable in a finite amount of time whose results are independent of ZFC [closed]

In On independence and large cardinal strength of physical statements we see that their are physical statements which are independent of ZFC, and even strong cardinal axioms. There were many answers, ...

2
votes

1
answer

168
views

### Internal operations on uncomputable functions

Is there know set of operations for which uncomputable functions are, let's name it down-unclosed? I mean a set of operations which takes two ( or more) uncomputable functions and return computable ...

4
votes

2
answers

771
views

### What things does ZFC not know if it knows?

The statement "ZFC $\vdash 0=1$" is independent of ZFC due to Goedel's second incompleteness theorem. That got me wondering, for what other statements $\phi$ is "ZFC $\vdash \phi$" independent of ZFC?
...

84
votes

4
answers

21k
views

### The enigmatic complexity of number theory

One of the most salient aspects of the discipline of number theory is that from a very small number of definitions, entities and axioms one is led to an extraordinary wealth and diversity of theorems, ...

1
vote

1
answer

1k
views

### Question about the validity of Michael Grossman, Robert Katz and Jane Grossman Non-Newtonian / Meta / Multiplicative Calculus [closed]

I am no mathematician by far, but I have studied diff. and integral calc and beyond, in my undergrad years. I recently came upon this book:
"The First Systems of Weighted Differential and ...

4
votes

1
answer

495
views

### Can adding Hilbert's epsilon to a theory, and then expanding that theories axiom schema to include the new language, cause it to become inconsistent?

Epsilon Calculus is a formalism developed by Hilbert adding his $\epsilon$ operator to predicate logic. $\epsilon x. A(x)$ is a term such that $\exists x.A(x) \implies A(\epsilon x.A(x))$. In can ...

1
vote

2
answers

749
views

### Can you remove all the extra arithmetic from ZFC (or other theories)?

Let $\mathbb{N}$ be the standard model of the natural numbers. For any statement in the language of arithmetic, we can translate into a statement in the language of set theory by asking if it is true ...

6
votes

1
answer

351
views

### What drawbacks are there to using NF(U) for category theory?

In category theory, you often run into what is known as "size" issues. That is, you run into the issue that the categories you try to define are too "big" to be sets, and so you need to use classes or ...

25
votes

2
answers

2k
views

### Can you have a type theory where there is type of all types?

Normally in a type theory, you can not have a type of all types, due to Girard's paradox. This is somewhat similar to how in set theory, you cannot have a set of all sets.
Therefore, usually you just ...

3
votes

1
answer

340
views

### Can you formulate a theory stating that a truth predicate does not exist for first order set theory?

A truth predicate for first order set theory would allow you to determine the truth of statements in first order set theory. A definition is given here.
My question is, can you formulate a statement ...

3
votes

0
answers

249
views

### A theory which denies the existence of a truth predicate

Is there a paper or other reference that explores the implications of a theory that denying the existence of a truth predicate in its own language (perhaps based on ZFC or PA)?
I know that a theory ...

2
votes

1
answer

626
views

### Is ZFC plus a truth predicate capable of variable substitution consistent?

Let us define a truth predicate that allows one to substitute in variables. For example, for a 3-ary predicate $p$ and sets $A$, $B$, and $C$, then $\text{true}(\ulcorner p \urcorner, A, B, C)$ (where ...

7
votes

2
answers

650
views

### What two ordinals are these (based on definable ordinals)?

Let $D$ be the set of definable ordinals. An ordinal s is definable if there is a predicate $p$ (in the language of (first-order) set theory), such that $p(x) \iff x=s$ for all $x$. This is definitely ...

7
votes

3
answers

499
views

### On structures that are not submitted to compatibility conditions

In mathematics, it seems that 100 % of the time, we deal with objects that have a number of different structures that are "compatible" with each other. For example,
a Lie group is a manifold and a ...

1
vote

0
answers

122
views

### Can $GPK^{+}_{\infty}$ + $AC_{WF}$ prove "$ZFC$ proves that the class of ordinals is not weakly compact for definable classes?

Consider the topological set theory $GPK^{+}_{\infty}$ +$AC_{WF}$, where $GPK^{+}_{\infty}$ is defined as follows (this from Olivier Esser's papers, "An Interpretation of the Zermelo-Fraenkel Set ...

7
votes

3
answers

304
views

### Automatic transfer of pointwise metric computations to bundle computations

$\newcommand{\M}{\mathcal{M}}$
$\newcommand{\N}{\mathcal{N}}$
$\newcommand{\deriv}[2]{\frac{d#1}{d#2}}$
$\newcommand{\sAverage}[1]{\langle#1\rangle} $
$\newcommand{\IP}[2]{\sAverage{#1,#2}}$
$\...

4
votes

0
answers

280
views

### the strength of saying "each sentence of true arithmetic has a recursive proof"

Let $PA_{\omega}$ be just like $PA$ except that $PA_{\omega}$-proofs can use any number of applications of the recursive $\omega$-rule.
The recursive $\omega$-rule allows the following:
For each ...

19
votes

2
answers

2k
views

### Which kind of foundation are mathematicians using when proving metatheorems?

Zermelo-Fraenkel set theory (with choice) is commonly accepted as the standard foundation of mathematics. It is a material set theory. For every two objects/sets $a,b$ one can ask whether $a=b$ or not....

9
votes

1
answer

856
views

### Is there any set theory $T$ such that $T$ plus true arithmetic is complete with respect to statements in set theory?

Is there an effective set theory $T$ such that $T + $$TA$ is consistient and complete. It should at least prove all theorems of $ZF$ true, so that it is a "standard" set theory. In particular, the ...

2
votes

0
answers

361
views

### The theory of frames and locales as elementary topology [closed]

In What is elementary geometry? (pdf) Alfred Tarski defined elementary geometry to be
that part of Euclidean geometry which can be formulated and established without the help of any set-...

8
votes

0
answers

289
views

### Gleason’s Theorem for non-separable Hilbert spaces: On a theorem of Solovay

Gleason’s theorem plays an important role in the foundations of
quantum mechanics. On the positive side it demonstrates how the probabilistic
structure of quantum theory follows from its logical ...

5
votes

1
answer

195
views

### Is it possible to prove concentration bounds from optional stopping theorem?

It is known that the optional stopping theorem from martingale theory is a very powerful theorem in probability theory in statistics.
I have heard of a probability course at Stanford where ...

17
votes

3
answers

5k
views

### Is there a compendium of the consistency strength between the most important formal theories?

Preliminar Notions:
A formal system is a tuple $(\Sigma,G,A,R)$ where $\Sigma$ is an alphabet (set of symbols), $G$ is a formal grammar on $\Sigma$ that generates a formal language $L$ (set of well ...