**2**

votes

**1**answer

159 views

### The set of $\Delta_1$ indices

Is the set of Godel numbers of $\Delta_1$ formulae itself $\Delta_1$ definable (i.e., computable)?

**5**

votes

**1**answer

204 views

### Companion of the pointclass of inductive sets

This question is about the notion of a companion for a Spector class, as defined in Moschovakis's book Elementary Induction on Abstract Structures.
I am interested in Spector classes on $\mathbb{R}$, ...

**8**

votes

**1**answer

417 views

### Fast-growing hierarchy and Turing machines

Is it possible to get an estimate of the size of a Turing machine computing $f_\alpha(n)$, for a given $\alpha$ (I am especialy interested in moderately large $\alpha$ like the ordinal of ...

**2**

votes

**2**answers

222 views

### Computability of finding roots in holomorphic functions.

Consider a holomorphic function $f: S \to \mathbb{C}$ where $S$ is a path connected open subset of $\mathbb{C}$ (not necessarily simply connected). Is it then possible to determine if $f$ contains a ...

**24**

votes

**1**answer

810 views

### Can a string's sophistication be defined in an unsophisticated way?

This question is about sophistication, a way of measuring the amount of "interesting, non-random information" in a binary string, which was proposed by Kolmogorov and others in the 1980s. I'll define ...

**13**

votes

**2**answers

509 views

### Minimal degree of polynomial vanishing on the variety of small degree.

My question is assume that we know that the degree of some irreducible variety is small does it possible to conclude that there exists polynomial of small degree vanishing on this variety.
Let us ...

**2**

votes

**1**answer

350 views

### Turing-Complete Cellular Automata and Sym(Z)

Does there exist a Turing complete, cellular automata with universe and alphabet $\mathbb{Z}$ such that the only allowable configurations are permutations of $\mathbb{Z}$? Formally, consider $\tau : ...

**1**

vote

**2**answers

280 views

### Satisfiability problem for FOL[<,R]

Let FOL[<,R] be the fragment of first-order logic enriched with two relational symbols < and R and the first-order axioms that say:
< is a strict partial order and R is an irreflexive and ...

**3**

votes

**1**answer

248 views

### Proof of the existence of hyperimmune-free degrees

In Classical Recursion Theory Vol.I by P.Odifreddi, section V.5 on the Tree Method, the proof for the existence of hyperimmune-frees involves the construction of a series of trees.
Some definitions ...

**4**

votes

**3**answers

308 views

### Existential quantification over regular predicates

A regular language over an alphabet $\Sigma$ is a subset of the set of all words over $\Sigma$ that can be accepted by some finite automaton. A regular language identifies a certain property of ...

**4**

votes

**1**answer

215 views

### Reference for a “recursive” fragment of infinitary logic?

Does anyone know of any texts or papers out there concerning properties of the fragment of $ \mathcal{L}_{\omega_1, \omega} $ in which the only admissable infinite conjunctions are those which are ...

**20**

votes

**4**answers

1k views

### Algorithmically unsolvable problems in topology

This question is inspired by a paper by B. Poonen that appeared on the arxiv some time ago: http://arxiv.org/abs/1204.0299. The paper gives a sample of algorithmically unsolvable problems from various ...

**3**

votes

**2**answers

1k views

### Is there an algorithm that can “reverse engineer” a Regular Expression?

Given a Regular language (represented as a black box to which one can apply inputs and get 0/1) Is there an algorithm that can find a finite deterministic automaton that produces that language?

**6**

votes

**3**answers

656 views

### computational complexity of primitive recursive functions

If we have a rewrite system for primitive recursive functions, which simplifies each term according to how the function was defined, then what is the computational complexity of this calculation? That ...

**3**

votes

**1**answer

293 views

### Diagonalization and classes of computable functions

Fix a standard effective listing $(\phi_e)_{e\in\omega}$ of the partial computable functions from $\omega$ to $\omega$. Let $\mathcal{C}$ be a class of computable total functions $\omega\rightarrow ...

**6**

votes

**1**answer

391 views

### Probability that a Turing machine will nontrivially reduce a real

For a fixed Turing machine $\Phi_e$, what is the probability that it will reduce a given real to some less complex, yet still non-computable real?
More precisely: It is known that the set of reals ...

**3**

votes

**2**answers

339 views

### Representation of μ-recursive functions

Can every μ-recursive function be defined using a single instance of the μ operator applied to a primitive recursive function?
According to Wikipedia, any μ-recursive function can be expressed as the ...

**22**

votes

**4**answers

17k views

### How large is TREE(3)?

Friedman, in http://www.math.osu.edu/~friedman.8/pdf/EnormousInt112201.pdf, shows that TREE(3) is much larger than n(4), itself bounded below by $A^{A(187195)}(3)$ (where $A$ is the Ackerman ...

**5**

votes

**1**answer

552 views

### Countable admissible ordinals

Jensen claimed that for any finite increasing sequence countable admissible ordinals $\omega= \alpha_0<\alpha_1\cdots <\alpha_n$, there is a real $x$ so that, for each $m\leq n$, $\alpha_m$ is ...

**3**

votes

**0**answers

95 views

### Is a parametric family which is universally consistent for multiple quantiles impossible?

Suppose I am dead-set on using Bayesian inference on independent and identically distributed data, but I'm lazy and insist on using a parametric likelihood function come what may. I'd be reassured to ...

**5**

votes

**1**answer

281 views

### Higher computability : Constructive ordinal and $\Delta^1_1$ predicates

Everything I know on this subject comes from Sacks book : "Higher recursion theory"
Let $\mathcal{O^Y}$ be the set of codes for ordinals constructive in $Y$.
We should have the result that $A ...

**6**

votes

**2**answers

905 views

### Busy Beaver - Proof for BB(2) = 4

Hi,
I need to prove the above claim.
I can show that $BB(2)\ge 4$ by building a turing machine,
but how can i show that $BB(2) \le 4$?
Searched a lot over the web, and saw that Rado proved it in ...

**4**

votes

**0**answers

273 views

### Difference between lambda-calculus with well-formed formulas vs properly-formed formulas

In S.C. Kleene's 1935 paper "$\lambda$-definability and recursiveness," he proves that all $\lambda$-definable functions are general recursive in the Herbrand-Godel sense and vice-versa. However, the ...

**3**

votes

**2**answers

707 views

### Kleene's fixed point theorem on recursive subsets of computable functions

I have a question about the possibility to apply/restate the Kleene fixed point theorem on recursive subsets of computable functions. I don't know if this is trivial and/or if related questions have ...

**4**

votes

**1**answer

454 views

### Infinite monkeys computing … triangle area?

I wonder if it is possible to specialize the question:
(a) What is the probability that a random Turing Machine program
will halt?, to: (b) What is the probability that a random Turing Machine
...

**10**

votes

**2**answers

494 views

### What is the computational-complexity-theoretic analogue of computable inseparability? For example, if P is not NP, are there disjoint NP sets with no separation in P?

Disjoint sets $A$ and $B$ are computably inseparable, if there
is no computable separating set, a computable set $C$ containing $A$ and disjoint from $B$. The
existence of c.e. computably inseparable ...

**12**

votes

**1**answer

622 views

### Does every feasible partial order relation on the natural numbers extend to a feasible linear order relation?

It is well known that every partial order on a set can be extended
to a linear order on that set. That is, for every partial order
$\lhd$ on a set $X$, there is a linear order $\prec$ on $X$ such
that ...

**56**

votes

**3**answers

5k views

### Can a group be a universal Turing machine?

This question was inspired by this blog post of Jordan Ellenberg.
Define a "computable group" to be an at most countable group $G$ whose elements can be represented by finite binary strings, with the ...

**2**

votes

**1**answer

186 views

### Is this c.e. set obtained via the Recursion Theorem?

Reading through a text on computability I came across a c.e. set defined as follows: Let $K=\lbrace x \in W_x \rbrace$ and let $f$ be a computable function. Then there exists $n \in \omega$ such that ...

**1**

vote

**1**answer

99 views

### Turing code numbers and c.e. sets.

Let $W_e$ be the c.e. set which is the domain of the p.c. function $\varphi_e$ and consider the equivalence $\sim$ such that $x \sim y$ if and only if $\varphi_x=\varphi_y$. I am wondering if $W_e$ ...

**1**

vote

**1**answer

342 views

### Infinite set with/without infinite c.e. subsets

Let $\varphi_e$ denote the p.c. function computed by the Turing Machine with code number $e$. I am looking at the set $M = \lbrace x : \neg (x < y)[\varphi_x=\varphi_y] \rbrace$. This set is ...

**3**

votes

**2**answers

259 views

### $\Delta_2$-inseparability?

Since long ago it is known the existence of non-recursive sets (i.e., non-$\Delta_1$), e.g., Halting problem. It is also known (firstly noticed by Trakhtenbrot, and deeply studied by Smullyan) the ...

**2**

votes

**1**answer

721 views

### What about the fastest-growing non-computable function ?

The Busy-Beaver trick provides a nice example of non-computable functions (let say from $\mathbb{N}$ to $\mathbb{N}$) which grows faster than any computable functions. But what can we say when we do ...

**9**

votes

**2**answers

346 views

### Can we represent computable functions by r.e. sets ?

As we know, if $f$ is a computable function, then every pre-image of a r.e. set under $f$ is also a r.e. set, i.e. $f^{-1}(X)$ is a r.e.set if $X$ is a r.e. set. So I want to know that if a function ...

**5**

votes

**4**answers

2k views

### Explicit expression for recursively defined functions

Consider a function $w(i)$, $i \in \mathbb{N}$, defined recursively by:
$w(0)=w(1)=1$, and
$w(i)={i}^{n}-\sum_{j=1}^{i-1}{i \choose j}w(j)$ for $i>1$.
Is it possible to write $w(i)$ out ...

**9**

votes

**3**answers

801 views

### Effective topos and computability in topological spaces

The classical computability theory taking place in $\mathbb{N}$, can be extended to more general spaces, like $T_0$ second countable topological spaces $(X, \mathcal{O}, v)$ where $\mathcal{O}$ is a ...

**6**

votes

**2**answers

474 views

### Natural statements independent from true $\Pi^0_2$ sentences

I am looking for sentences in the language of first order arithmetic ($0,1,+,\cdot,\leq$) which are independent from $\Pi^0_2$ consequences of true arithmetic $\Pi^0_2\text{-}\mathsf{Th}(\mathbb{N})$. ...

**2**

votes

**1**answer

324 views

### On families of finite graphs with undecidable first-order theory

It is well known that the first order theory of graphs (i.e., an irreflexive and symmetric "edge" relation on a set) is undecidable. The same holds for the first order theory of finite graphs.
I am ...

**0**

votes

**1**answer

151 views

### any given c.e.set has number M whose power bounds the corresponding elements of S?

For S ,any given c.e.set,does there exist a M (integer) and a partially computable function outputing every element of S the c.e.set ,such that $\forall x\in S,\exists n x=f(n)$ and $x=f(n)\leq ...

**8**

votes

**3**answers

1k views

### Undecidable problems in geometry

Are there any (many) algorithmically undecidable problems in computational (combinatorial/discrete) geometry?
Update: the Wang tiles answer the question with "any". (I have somewhat overlooked to ...

**19**

votes

**3**answers

2k views

### Inverse Ackermann - primitive recursive or not?

I wanted to put this originally on math.stackexchange, since I considered it to be a straightforward question and probably a fairly known fact. After I failed to solve the problem, I browsed through ...

**1**

vote

**3**answers

439 views

### Variant of the usual proof method for undecidability of the halting problem

This is a largely a question of pedagogy/references, though I may have overlooked some nuance of actual mathematics.
I am planning to introduce the concept of Turing machines and the halting problem ...

**25**

votes

**3**answers

1k views

### Is it decidable whether or not a collection of integer matrices generates a free group?

Suppose we have integer matrices $A_1,\ldots,A_n\in\operatorname{GL}(n,\mathbb Z)$. Define $\varphi:F_n\to\operatorname{GL}(n,\mathbb Z)$ by $x_i\mapsto A_i$. Is there an algorithm to decide whether ...

**24**

votes

**15**answers

4k views

### What's a magical theorem in logic?

Some theorems are magical: their hypotheses are easy to meet, and when invoked (as lemmas) in the midst of an otherwise routine proof, they deliver the desired conclusion more or less ...

**13**

votes

**1**answer

652 views

### Are wild problems related to undecidable ones?

In representation theory, there is a well-known notion of a wild classification problem (such problems have been discussed often on this forum, for example, here). In logic, there is a notion of an ...

**0**

votes

**1**answer

651 views

### When may Function (meromorphic) be expanded as power series with coefficients of integers

Let F be meromorphic function,with what properties may it expanded as power series with coefficients of integers in such a form:
$$F=\sum_0^{\infty}a_i x^i,a_i\in \mathcal{N} \bigcup 0,\exists M ...

**3**

votes

**1**answer

296 views

### Paths in Kleene's O and deciding $\Pi^0_1$ sentences

This question comes from the
Wikipedia article on Kleene's O and a previous Math Overflow question.
The claim in Wikipedia that I have a question about is the second sentence in the following quote.
...

**5**

votes

**1**answer

514 views

### Machine model for primitive recursion?

General computable functions can be described either functionally (in terms of closure of
the coordinate functions, constant functions, composition, primitive recursion, and $\mu$-recursion), or in ...

**3**

votes

**1**answer

424 views

### Hyperarithmetic statements decidable by induction up to a recursive ordinal

The first version of this question received a helpful answer but was too vague to fully convey what I intended. I hope this version remedies that problem. For any hyperarithmetic set of integers $S$, ...

**6**

votes

**4**answers

258 views

### What sets are “decidable from competing provers”?

Let $n$ be a member of $\omega$, which the omnipotent provers $Y$ and $N$ know. A Turing machine will be run starting with the $n$ already inputted, and the machine can have a natural number inputted ...