All Questions
13 questions
9
votes
2
answers
426
views
Can local $0^\#$ exists in L?
Assume $0^\#$ exists and there is an inaccessible cardinal.
Are there two transitive sets $M,N$ s.t. $M\in N,M\vDash ZF+V=L[0^\#],N\vDash ZF+V=L$?
- 20.9k
6
votes
1
answer
571
views
Parameter-free effective cardinals
In the paper "Effective cardinals and determinacy in third order arithmetic" by Juan Aguilera, effective cardinals is defined.
I'm curious about its little variation, parameter-free ...
- 2,031
13
votes
1
answer
2k
views
Are some interesting mathematical statements minimal?
Gödel's set $\mathrm{L}$, of constructible sets, decides many interesting mathematical statements, as the Continuum hypothesis and the Axiom of Choice.
Are some interesting mathematical questions, ...
- 236k
2
votes
1
answer
243
views
Is stable ordinals in non-well-founded model the same as well founded models?
Let $BST$ be the axiom system $KP$ - $\Delta_0$ collection. For an ordinal $\alpha$, we say that $\alpha$ is $\varphi$-$\Sigma_n$-stable, if there is a $\beta>\alpha$ satisfies the formula $φ$ such ...
- 2,031
1
vote
1
answer
249
views
Recursively inaccessible ordinals and non locally countable ordinals
This answer seems to imply that: for an ordinal $\alpha$, to be recursively inaccessible (i.e. $\alpha$ is admissible and limit of admissible) implies to be not locally countable (i.e. $L_\alpha \...
- 9,902
8
votes
3
answers
538
views
Elementary countable submodels in Gödel's universe
By the downward Lowenheim-Skölem theorem we can find two countable ordinals $\alpha < \beta$ such that $L_\alpha \prec L_{\omega_1}$ and $L_\beta \prec L_{\omega_1}$. That is, $L_\alpha$ and $L_\...
- 39.8k
5
votes
1
answer
407
views
Height of diamond
Assume $V=L$.
Let $\alpha$ be the least ordinal such that there is a $\Diamond_{\omega_1}$-sequence in $L_\alpha$.
It's obvious that $\omega_1 < \alpha < \omega_2$.
Do we have some better ...
- 12.9k
7
votes
2
answers
518
views
Can countable ordinals start gaps of every order in the constructible universe?
Define "$\alpha$ starts a gap of order $n+1$ and length $\beta$" iff $\mathcal P^n(\omega)\cap (L_{\alpha+\beta}\setminus L_\alpha)=\emptyset\land\forall\gamma\in\alpha: L_\alpha\setminus L_\...
- 421
2
votes
0
answers
240
views
When is a $\Sigma_n$ Skolem hull a proper submodel?
For $M$ an amenable structure and $X \subset M$, the $\Sigma_n$ Skolem hull of $X$ is a $\Sigma_n$-elementary submodel of $M$. That is, as presentend in Devlin, Constructibility, pp. 85-88, for $h_n$ ...
- 531
8
votes
1
answer
627
views
Is $\mathsf{ZFC+V=L}$ consistently $\omega$-complete?
This was previously asked and bountied on MSE:
For brevity, let $T$ be $\mathsf{ZFC+V=L}$.
Say that an extension of $\mathsf{ZFC}$ is $\omega$-complete iff it has exactly one $\omega$-model up to ...
- 20.9k
3
votes
1
answer
325
views
If we have a class like $L$ but allowing a set number of unbounded quantifiers, is it strict superset of $L$?
The definition of $L$ only permits bounded quantifiers. If we allow a certain number of unbounded quantifiers, does this result in a strict superset of $L$? For example:
$$
\operatorname{Def}^{\...
- 6,353
1
vote
1
answer
228
views
Is there a 'Constructible Universe' that is a submodel of a non-transitive model of $ZF$?
In Barwise's book, Admissible Sets and Structures, the following statement is made (on pg. 8):
"...if $ZF$ is consistent, so is $ZF$ + "There is no transitive model of $ZF$"
He mentions this fact ...
- 39.8k
10
votes
2
answers
737
views
Constructible models of New Foundations?
Hi all! Is there anything like Gödel's constructible universe for New Foundations?
More precisely, I would like a process for taking a model $M$ of NF, and using it to build a model $L \subseteq M$ ...
- 601