All Questions
Tagged with co.combinatorics computability-theory
28 questions
45
votes
5
answers
64k
views
How large is TREE(3)?
Friedman, in _Lectures notes on enormous integers shows that TREE(3) is much larger than n(4), itself bounded below by $A^{A(187195)}(3)$ (where $A$ is the Ackerman function and exponentiation ...
- 257
4
votes
4
answers
472
views
Automatically generating combinatorial conjectures
It very often happens that one reduces a problem to a bunch of combinatorial data, and need to sift through this data for patterns, which form conjectures on which to do "real" mathematics. ...
12
votes
1
answer
544
views
Is the set of cube-free binary sequences perfect?
This question is inspired by this one. In that thread, it's established that there are uncountably many cube-free infinite binary strings (where $x \in 2^{\omega}$ is cube-free iff $\forall \sigma \...
- 1,329
12
votes
4
answers
4k
views
reversible Turing machines
Hello,
Let T be a Turing machine such that
1) it operates on the alphabet {0,1},
2) its set of states is A
3) the language it accepts is $L$ .
Does there exists a Turing machine S which also ...
- 236k
29
votes
2
answers
1k
views
Determining if a rational function has a subtraction-free expression
This question was first asked by Mehtaab Sawhney in Alex Postnikov's combinatorics class.
Given a rational function $F=P(x_1,...,x_n)/Q(x_1,...,x_n)$ with (say) integer coefficients, it is often of ...
- 91.8k
10
votes
2
answers
595
views
Transfinite algorithms
The Ford-Fulkerson algorithm is a classic algorithm that computes the maximum flow in a network. It is well-known that if irrational arc capacities are allowed, the algorithm does not necessarily ...
- 211
0
votes
1
answer
184
views
Combinatorially defined effectively closed set
Is there a combinatorially defined, nonempty effectively closed set $Q\subseteq 2^\omega$ such that all members of $Q$ are incomputable?
Combinatorially defined means that the definition of $Q$ does ...
- 236k
46
votes
3
answers
3k
views
Does an existence of large cardinals have implications in number theory or combinatorics?
Does an existence of large cardinals have implications in more down-to-earth fields like number theory, finite combinatorics, graph theory, Ramsey theory or computability theory? Are there any ...
- 4,713
5
votes
0
answers
301
views
The expressiveness of functions computable on trees
Motivation:
Let's define a function computable on a $k$-ary tree as a function composed with simpler computable functions defined at each node such that a function of this kind defined on a binary ...
CommunityBot
- 1
26
votes
6
answers
9k
views
The problem of finding the first digit in Graham's number
Motivation
In this BBC video about infinity they mention Graham's number. In the second part, Graham mentions that "maybe no one will ever know what [the first] digit is". This made me think: Could ...
4
votes
1
answer
160
views
Is sum-balanceability computable?
Let $\mathbb{N}$ denote the set of positive integers, and let $G=(V,E)$ be a finite simple, undirected graph. Given $f:V\to \mathbb{Z}$ we define the neighborhood
sum function $\mathrm{nsum}_f:V\to\...
- 32.1k
13
votes
0
answers
257
views
Is the set of power matrices decidable?
Let $\text{Mat}(n\times n,\mathbb{Z})$ denote the collection of integer $n\times n$ matrices. We say $M\in \text{Mat}(n\times n,\mathbb{Z})$ is a power matrix if there is an integer $k>1$ and a ...
- 48.9k
5
votes
0
answers
158
views
The set of homogeneous solutions of a clopen contains an hyperarithmetical set
In the context of Galvin-Prikry generalization of Ramsey's theorem, I read in a couple of papers ([1],[2]) that Solovay [3] proved that if $P$ is a clopen of $[\mathbb{N}]^{\mathbb{N}}$ then the set ...
- 4,589
4
votes
0
answers
164
views
Tileability and computabilty
Let $n>2$ be an integer. We consider $n$ pairs $(x_1,y_1),\dotsc,(x_n,y_n)$ in $\mathbb{N}^2$, and the polygon defined by drawing a straight line from $(x_k, y_k)$ to $(x_{k+1},y_{k+1})$ and from $(...
- 48.9k
1
vote
1
answer
243
views
A possible minimal aperiodic set of corner Wang Tile
From one of my previous question Aperiodic set of corner Wang Tile (although it is put on hold), I realize there is a systematic way to construct aperiodic corner type of Wang tile from edge type ...
- 4,713
8
votes
1
answer
351
views
How long does the slow inefficient algorithm for computing the product in classical Laver tables take?
Let $(A_{n},*)$ denote the $n$-th classical Laver table. Let
$X_{n}$ be the set of all finite sequences of elements from $A_{n}$.
Define a function $E_{n}:X_{n}\rightarrow X_{n}$ by letting
$E_{n}((...
CommunityBot
- 1
7
votes
0
answers
704
views
A way to smooth out the log* function?
I have seen here and there discussions about what is the "correct" way of extending the Ackermann function to the reals (the same way the Gamma function extends the factorial function to the reals). ...
CommunityBot
- 1
12
votes
1
answer
684
views
Continuous functions and 2-bushy trees
The following problem was asked by Joe Miller in the fall of 2010 at a bar in Madison.
A subtree $T \subseteq 4^{< \omega}$ is $2$-bushy if for some node $\sigma \in T$, every node above $\sigma$ ...
- 148k
0
votes
0
answers
105
views
Counting path generating sentences in a specific formal language
Given a formal grammar of a language or an Turing machine of the language, can we count the path that generating sentences of the language?
For example, we know that if the grammar is context-free ...
- 38.6k
5
votes
1
answer
213
views
Aperiodic set of corner Wang Tile [closed]
There is quite some reference on aperiodicity of the edge-type of Wang Tile. But I could not yet find aperiodic corner type of Wang Tiles... Could someone provide me some instances (better with ...
CommunityBot
- 1
2
votes
3
answers
987
views
An established proof in Wang Tile which I doubt
When I was reading the paper:
Wang, Hao. "Notes on a class of tiling problems." Fundamenta Mathematicae 82.4 (1975): 295-305.
from http://matwbn.icm.edu.pl/ksiazki/fm/fm82/fm82119.pdf
I could not ...
- 13.4k
3
votes
2
answers
297
views
Conjecture of a subset of Wang tile which might be decidable
From the two papers proving the undecidability of Wang tile in 1966 by Berger and in 1971 by RM Robinson, the tiles used in proving undecidability has a general common feature:
The left color and ...
CommunityBot
- 1
1
vote
1
answer
631
views
relationship between corner tile and edge tile of wang tile
It is clear that any corner type of Wang Tile could be converted to edge type of Wang Tile by defining the edge color according to the corner color.
However, could we convert edge type of Wang Tile ...
- 236k
3
votes
1
answer
509
views
Application of Combinatorics, Logic and computability theory in physical science: Tiling of Wang Tile with proportionality
The original problem of Domino Tiling and Wang Tile has great theoretical interest on computability theory... However, the great emerging problem on application of Wang Tile in material science and ...
CommunityBot
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3
votes
3
answers
575
views
Infinite Partitions of the Primes and Sums of Reciprocals (Revised)
I have revised my original post. The questions I asked there were not well-put or even thought through. I don't want to delete, however, since some of the comments may be of interest to other MO users....
CommunityBot
- 1
2
votes
1
answer
224
views
String transformer : Polynomial time approximation schemes?
A program P takes a string as an input and returns a string of same length as output.
Q Given two strings A and B how fast can a program tell weather string B cannot be obtained by a recursive ...
CommunityBot
- 1
17
votes
1
answer
960
views
Polynomial-time algorithm to compare numbers in Conway chained arrow notation
I am looking for a polynomial-time algorithm which, given a character string containing two numbers in Conway's chained arrow notation for large numbers, indicates whether the first number is less ...
CommunityBot
- 1
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 ...
- 24.8k