All Questions
54 questions
5
votes
0
answers
158
views
If $\omega_1$ is not inaccessible in $L$, how hard can it be to find a non-measurable $\Sigma^1_3$ set of reals?
In his wonderfully titled paper Can you take Solovay's inaccessible away? Shelah showed that if every $\mathbf{\Sigma}^1_3$ set of reals is Lebesgue measurable, then $\omega_1$ is an inaccessible ...
- 12.5k
2
votes
1
answer
161
views
Are Cohen Generics Minimal Covers?
Are Cohen generics (in $2^\omega$) minimal covers?
I'm ultimately interested in this question for some more effective notion of forcing but I realized I wasn't sure how to show this even assuming full ...
- 3,029
1
vote
0
answers
108
views
Name For Effective Cantor-Bendixsonish Derivitive
When dealing with a tree (substring closed subset of $\omega^{< \omega})$ a useful operation will frequently be to remove any nodes with finite ordinal rank (i.e., all nodes whose extensions on the ...
- 3,029
2
votes
0
answers
118
views
Uniformization and functions on Turing degrees
Assuming Martin's Conjecture on functions between Turing degrees, is AD + DC consistent with existence of an $f:\mathcal{D}_t → \mathcal{D}_t$ of rank $Θ$ ?
$\mathcal{D}_t$ is the set of Turing ...
- 7,545
4
votes
3
answers
406
views
Hyperarithmetically least elements in $\Pi^1_1$ sets
My question is: Do we have a hyperarithmetically $\le_H$-least real in any $\Pi^1_1$ set? That is
Question. Suppose that $A$ is a non-empty $\Pi^1_1$ set. Then can we find a real $a\in A$ such that $...
- 3,042
3
votes
1
answer
134
views
A $\Pi^0_1$ class of $\alpha$-subgenerics for all $\alpha < w_1^{CK}$
In Harrington's mimeographed notes (see here) solving McLaughlin's conjecture he builds reals $f \in \omega^\omega$ which have the property of being $\alpha$-subgeneric defined as follows. He does ...
- 3,029
1
vote
1
answer
98
views
Intersection of (relativized/preimage) measure 0 with every hyperarithmetic perfect set
Given a perfect tree $T$ on $2^{<\omega}$ viewed as a function from $2^{<\omega}$ to $2^{<\omega}$ define the measure of a subset of $[T]$ to be the measure of it's preimage under the usual ...
- 3,029
4
votes
1
answer
533
views
Complexity of |a| < |b| for ordinal notations?
What is the complexity (e.g. is it $\Sigma^0_1$, arithmetic, fully $\Pi^1_1$) of the relation $|a| < |b|$ given two notations $a, b \in \mathscr{O}$ (Kleene's O)?
What about the case where only one ...
- 3,029
2
votes
1
answer
118
views
$\Pi^0_2$ singleton forming minimal pair with $0''$
Is there a $\Pi^0_2$ singleton that forms a minimal pair with $0''$? That is, is there a set $X$ such that $X$ is the unique solution to $\forall x \exists y \phi(X|_y, x)$, $X$ and $0''$ are ...
- 3,029
4
votes
1
answer
142
views
Does the set of infinite random strings satisfy an analogue of immune sets?
Let $K(x)$ denote the Kolmogorov complexity of a finite binary string $x$. A finite binary string $x$ is called Kolmogorov random if $K(x) \geq |x|$. And an infinite binary sequence is called Martin-...
- 4,597
6
votes
0
answers
117
views
Reverse mathematics of Banach-Mazur games
Given $\mathcal{A}\subseteq\omega^\omega$, the Banach-Mazur game with payoff set $\mathcal{A}$ consists of players $1$ and $2$ alternately playing nonempty finite strings of naturals with player $1$ ...
- 21.2k
6
votes
0
answers
151
views
Complexity of constructive arithmetical truth vs second order arithmetic
Let us say that an arithmetic statement is constructively true iff it is realized by a computable function under Kleene's function realizability. Does the set of constructively true (first order) ...
- 7,545
3
votes
0
answers
203
views
Set-theoretic hierarchy using the uniqueness quantification
Has an equivalent of the set-theoretic hierarchies (arithmetical, hyperarithmetical, Levy etc.) that uses the uniqueness quantification, $\exists !$ (and its dual, $\neg\exists!\neg$) been studied ...
- 531
7
votes
0
answers
304
views
Which countable ordinals are "Barwise compact" for $\mathcal{L}_{\infty,\omega_1}$?
Barwise compactness says (as a special case) that whenever $\alpha$ is countable and admissible, $T\subseteq\mathcal{L}_{\infty,\omega}\cap L_\alpha$ is $\alpha$-c.e., and every subset of $T$ which is ...
- 21.2k
7
votes
0
answers
471
views
Infinite time Turing machines, semi-decidable sets and descriptive set theory
Definition A set of reals $A$ is said to be ittm-eventually-semi-decidable if there is an Infinite Time Turing Machine programme $P_e$ so that $x\in A$ iff $P_e(x)$ has converged on “1” on its ...
- 4,839
3
votes
0
answers
223
views
Bimodal determinacy logic for Borel games
This question is intended to be a first step towards answering this old question of mine.
Let $K$ be the set of pairs $(\Sigma,\Pi)$ of quasistrategies, in the usual sense of games on $\omega$, for ...
- 21.2k
2
votes
0
answers
258
views
Can we have a "very strong" cone phenomenon in the Turing degrees (and a related question)?
By Borel determinacy + Martin's cone theorem, for every countable fragment $\mathcal{A}$ of $\mathcal{L}_{\omega_1,\omega}$ there is a turing degree ${\bf c}$ such that for all ${\bf d}\ge_T{\bf c}$ ...
- 21.2k
8
votes
1
answer
514
views
How big is the least non-$\Sigma^1_1$-pointwise-definable ordinal?
There's a large countable ordinal which has cropped up (as a lower bound!) in a computable structure theory problem I'm playing with. At present I don't really understand how big it is, and I'm ...
- 21.2k
9
votes
1
answer
495
views
Can two versions of $\omega_1^{CK}(\mathsf{Ord})$ ever coincide?
The goal of this question is to fill in the gap in this old answer of mine.
For a transitive set $M$, thought of as an $\{\in\}$-structure, we define the following ordinals (this is not the notation ...
- 21.2k
10
votes
1
answer
287
views
Complexity of the set of models of TA
Recall that the theory of true arithmetic $TA$ is the theory of standard model of arithmetic $\mathcal N$. I am interested in the complexity of the set of countable models of $TA$ in the lightface or ...
- 515
9
votes
1
answer
610
views
Does every cofinal branch through Kleene's O compute true arithmetic?
My question concerns cofinal branches through Kleene's $O$, which is a set of natural numbers and a computably enumerable relation $<_O$ on this set that provides
ordinal denotations for any ...
- 236k
4
votes
0
answers
177
views
When is validity definable in $L_\alpha$?
Below, $\alpha$ is a countable p.r.-closed ordinal $>\omega$.
Let $\mathcal{L}_\alpha=\mathcal{L}_{\infty,\omega}\cap L_\alpha$ (note that this is not the same as $\mathcal{L}_{\omega_1,\omega}\...
- 21.2k
2
votes
1
answer
184
views
Detecting comprehension topologically
This question basically follows this earlier question of mine but shifting from standard systems of nonstandard models of $PA$ to $\omega$-models of $RCA_0$. For $X$ a Turing ideal we get the map $c_X$...
- 21.2k
0
votes
1
answer
87
views
Kurtz randomness and supermartingales with infinite *limit*
Suppose you replace the usual success conditions for a supermartingale (lim sup is infinite) with the requirement that the actual limit is infinite, e.g. a supermartingale $B$ succeeds on $X \in 2^\...
- 3,029
7
votes
1
answer
490
views
"Robinson arithmetic" for (some) levels of $L$?
I'll write "$\mathcal{L}_\alpha$" for the fragment $\mathcal{L}_{\infty,\omega}\cap L_\alpha$.
Say that a countable admissible $\alpha$ is Robinsonian if there is some sentence $\varphi\in\mathcal{L}...
- 21.2k
6
votes
1
answer
503
views
Regularity properties of Turing-invariant and arbitrary sets of reals
The question whether Turing determinacy implies $AD$ is a well-known open problem. I was wondering if anything is known about the following analogous question:
Let $\Gamma$ be a regularity property (...
- 391
5
votes
0
answers
196
views
A slight extension of Sacks theorem
Sacks proves the following theorem first.
Theorem 1: If $\alpha$ is a countable admissible ordinal, then there is a real $x$ so that $\omega_1^x=\alpha$.
Anyone knows who proves the following ...
- 4,201
7
votes
1
answer
303
views
Variously pointed closed sets
A tree $A\subseteq \omega^{<\omega}$ - possibly with dead ends - is pointed iff every path $p\in[A]$ has $p\ge_TA$. This lifts to two distinct notions of pointedness for closed sets in Baire space: ...
- 21.2k
6
votes
0
answers
806
views
A strong plus-one hypothesis
To make this more easily readable, I'll start with the question and then give the explanation/motivation.
Question. Is the following principle (or its weakening, with "for every real $r$" ...
- 21.2k
7
votes
0
answers
284
views
Co-cones in the Turing degrees
Let the cocone of a Turing degree ${\bf d}$ be the set $cc({\bf d}):\{{\bf c}: {\bf c}\not\ge_T {\bf d}\}$. I'm curious what's known about the various partial orders (isomorphic to ones) of the form $...
- 21.2k
9
votes
1
answer
739
views
Can the Turing degrees be linearly ordered?
Assuming the axiom of choice, every set can be linearly (indeed, well-) ordered. However, without choice this can fail, as witnessed most drastically by the consistency of amorphous sets. More ...
- 21.2k
5
votes
1
answer
292
views
A game with boldface strength
This is a problem which has been bothering me for a while now; it doesn't seem inherently too hard, but I haven't been able to make any real headway, so I'm putting it out in the open since at this ...
- 21.2k
15
votes
1
answer
616
views
Does Kechris' conjecture contradict both parts of Martin's conjecture, or just part 1?
By Kechris' conjecture (KC) I mean the assertion that Turing equivalence $\equiv_T$ is a universal countable Borel equivalence relation.
On the other hand, Martin's conjecture (MC) is a long-lasting ...
- 151
6
votes
1
answer
433
views
Reference request: a version of $\Sigma^1_1$ bounding for structures
There's a (fairly basic) fact I want to use in a paper I'm writing; it's not entirely trivial, so I don't feel comfortable just stating the result and moving on, but I don't have a citation for it. ...
- 21.2k
8
votes
0
answers
451
views
The cone property in the enumeration degrees
A Borel partial order is the partial order corresponding to a Borel preorder of some Polish space. For example, the Turing and enumeration degrees, $\mathcal{D}$ and $\mathcal{E}$ respectively, are ...
- 21.2k
8
votes
1
answer
432
views
Which reals are "hyperarithmetic modulo ordinals"?
The context for this question is the theory ZFC + a measurable cardinal, although answers not in this context would also be interesting to me.
In a project I'm working on, the following class of ...
- 21.2k
9
votes
0
answers
471
views
(A little bit) Beyond the E-recursive
The E-recursive functions are a particular generalization of classical recursion theory to the entire set-theoretic universe, $V$. They are defined via a schemes: see Sacks' $E$-recursive intuitions. ...
- 21.2k
14
votes
2
answers
719
views
Woodin on Posner-Robinson for the hyperjump and sharp
The Posner-Robinson theorem states that, if $X$ is noncomputable, there is some $G$ such that $X\oplus G=G'$; that is, even though genuine jump inversion only works above $0'$, every (nontrivial) $X$ ...
- 21.2k
10
votes
1
answer
411
views
The least admissible above a dominating real
Let $\mathbb{P}$ be the usual forcing which adds a dominating real: conditions in $\mathbb{P}$ are pairs $(p, f)$ with $p:\omega\rightarrow\omega$ finite partial and $f:\omega\rightarrow\omega$ total, ...
- 21.2k
2
votes
2
answers
245
views
Precise interpretability strength of $\mathcal P_{DF}(\omega)$ and $L_{\omega_1^{CK}}$
I am curious about the relationship between the definable power set of $\omega$ and the $\omega_1^{CK}$th level of the constructible sets $L$.
In short, $\omega_1^{CK}$ is the least nonrecursive ...
- 29
7
votes
2
answers
657
views
Topological tameness beyond the Gandy-Harrington topology
The Gandy-Harrington topology on $\omega^\omega$ is the topology generated by all lightface $\Sigma^1_1$ sets; that is, all sets which are continuous-in-the-usual-sense images of $\omega^\omega$.
...
- 21.2k
6
votes
1
answer
259
views
Degree of unsolvability of finding a open approximation to a Borel set, given its Borel code
It is well known that every Borel set has the property of Baire. That is, for every Borel set $B$, there is an open set $U$ and a sequence of dense open sets $D_n$ such that for every $x\in \cap_n ...
- 1,601
14
votes
2
answers
1k
views
Does Turing determinacy imply full determinacy?
The axiom of Turing determinacy is a weakening of the full axiom of determinacy, $AD$, in which only games with payoff sets which are $\equiv_T$-invariant are demanded to be determined.
In "...
- 21.2k
15
votes
1
answer
1k
views
Higher recursion theory and reverse mathematics: What is to $\Pi^1_1$-$CA_0$ as $RCA_0$ is to $ACA_0$?
There is an extremely rich and well-understood analogy between "recursively enumerable" and "$\Pi^1_1$" – indeed, this is the starting point of metarecursion theory, and $\alpha$-...
- 21.2k
11
votes
1
answer
441
views
Concerning Silver's result
Jack Silver proved that if $x$ is a real so that every $x$-admissible ordinal is a cardinal in $L$, then $0^{\sharp}$ exists.
I wonder whether various weaker or stronger versions of Silver's result ...
- 4,201
7
votes
1
answer
1k
views
Is the equivalence between a $\Sigma^0_1$ and a $\Pi^0_1$ formula defining the same recursive set provable in a sufficiently strong arithmetic ?
Let $A$ be a recursive set. $A$ is recursively enumerable, so $A$ may be defined by a $\Sigma^0_1$ formula, i.e. by $\exists \overrightarrow{a} \phi (\overrightarrow{a}, n)$, where $\phi$ contains no ...
- 73
32
votes
1
answer
2k
views
Godel on recursion-theoretic hierarchies
At the end of his excellent article, "The Emergence of Descriptive Set Theory" (http://math.bu.edu/people/aki/2.pdf), Kanamori writes:
"Another mathematical eternal return: Toward the end of his ...
- 21.2k
4
votes
1
answer
319
views
Complexity of winning strategies for open games (for open player)
If $G\subseteq\omega^{<\omega}$ is a computable clopen game, then $G$ has a winning strategy which is hyperarithmetic $(\Delta^1_1)$, by an inductive ranking process. The key observation here is ...
- 21.2k
5
votes
1
answer
231
views
When do substructures have computable copies?
Say that a class $\mathcal{C}$ of countable first-order structures in some finite signature has the effective substructure property if $\mathcal{C}$ is closed under isomorphism and whenever $A\in \...
- 21.2k
9
votes
1
answer
334
views
Ensuring nonempty lightface Borel sets have elements via theories of second-order arithmetic
This question is an outgrowth of this MathSE question: https://math.stackexchange.com/questions/276068/members-of-lightface-borel-sets.
A Borel set $X\subseteq 2^\omega$ is a member of the smallest ...
- 21.2k