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Questions tagged [undecidability]

Questions about problems which cannot be solved by any Turing machine.

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3
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1answer
375 views

Undecidability of two Turing machines acting the same way on an input

So I need to find a reduction to the (undecidable) problem of deciding if two Turing machines $M_1$ and $M_2$ behave the same way on an input $x$. "Behaving the same way" is defined like this: $M_1$ ...
0
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0answers
183 views

Proving sets of regular expressions and context free grammars are decidable [duplicate]

Consider below languages: $L_1=\{<M>|M$ is a regular expression which generates at least one string containing an odd number of 1's$\}$ $L_2=\{<G>|G$ is context free grammar which ...
0
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1answer
249 views

In the reduction from HALT to ALLHALT, why does the constructed Turing machine loop indefinitely when the inputted Turing machine rejects?

Let HALT be the language $\{\langle M, w\rangle : M\text{ is a TM that halts on }w \}$. Let ALLHALT be the language $\{\langle M\rangle : M\text{ is a TM that halts on all inputs}\}$. Use a reduction ...
1
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1answer
61 views

TM1 accepts w1 vs TM1 halts on w1

What is difference between following two problems, their decidability and recognizability status: Given Turing Machine TM "accepts" given string w. Given Turing Machine TM "halts on" given string w. ...
0
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1answer
138 views

is the empty language L = ∅ a subset of every languages?

I need to show false the following claim Every language L which is a subset of $A_{TM}$ ($L \subseteq A_{TM}$) is undecidable. For this, I wish to use the empty language L = ∅ (I know is decidable)...
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2answers
606 views

if A is decidable then B is decidable too

Assume that a language A is reducible to language B. The claim is true? if A is decidable then B is decidable too. The correct answer is: This claim is wrong. If A is e.g. the empty language (...
6
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1answer
1k views

Prove that the class of CFG languages that are closed under reversal is undecidable

Note The wording of the title may be a bit vague, but I'm not asking if CFLs are closed under reversal. Please see below. Problem Description Given a word $w$, define $w^{r}$ to be its reversal. ...
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2answers
261 views

A special case of subset sum

I came across the following problem in my complexity-theory course: Given a set of numbers $A := \{a_1, \dots, a_n\} \subset_{\mathrm{finite}} \mathbb{N}$ and a number $b$ also in $\mathbb{N}$ such ...
1
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1answer
403 views

Is it decidable whether a given Turing machine moves its head more than 481 cells away from the left-end marker, on input ε?

So, while reading some problems on decidability, I came across the following resource: https://www.isical.ac.in/~ansuman/flat2018/tm-more-undecidable.pdf Here, on page no 12, it is written that the ...
1
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1answer
27 views

Is checking if the length of a C program that can generate a string is less than a given number decidable?

I was given this question: Komplexity(S) is the length of the smallest C program that generates the string S as an output. Is the question "Komplexity(S) < K" decidable? With respect to ...
1
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1answer
50 views

Is the problem of deciding whether two programs have the same semantics decidable?

If I have program and I want to check whether other programs have the exact same semantics or not, could I always build a machine that could make that decision? This is a question relevant to ...
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0answers
39 views

A TM that doesn't decide Σ*, and a TM that doesn't decide the empty set?

I was wondering if it was possible to create a TM that semi-decides (but doesn't decide) either of the following two languages: $\emptyset$ $\Sigma^{*}$ I assume for 2, a one-state TM that just ...
1
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1answer
76 views

TM decidable or undecidable problem

Problem: Given a TM $M$ on the alphabet $\{0,1\}$, determine if there is some input on which $M$ executes for at least 5 steps. Is this problem decidable or not? To check if the problem is ...
1
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2answers
77 views

Why can't we prove decidability of $L= \{ \langle M \rangle : M$ accepts $ \epsilon \}$ with a configurations graph?

Since every deterministic Turing Machine can be translated to a graph of configurations such that $M$ accepts a word $w$ iff there is a path from the initial configuration that matches $w$ to an ...
2
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1answer
3k views

Is it decidable "Given a TM M, whether M ever writes a non blank symbol when started on the empty tape."

I came across below problem in this pdf: Given a TM M, whether M ever writes a non blank symbol when started on the empty tape. Solution given is as follows: Let the machine only writes blank ...
0
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2answers
225 views

Halting problem vs Universal Language

Wikipedia defines halting set as follows: $H = \{(i, x) |$ program $i$ halts when run on input $x\}$ Ullman defines universal language as follows $U = \{(M, w) |$ Turing machine $M$ accepts $w\}$ ...
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3answers
504 views

Is it possible that the subtraction between two undecidable languages is regular?

If $L_1$ and $L_2$ are both non-decidable languages (Not decidable, so can be SD or $\lnot$SD), is it possible that $L_1-L_2$ is regular and $L_1-L_2\neq\phi$, where $\phi$ is the empty set? What's ...
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2answers
165 views

Why Rice theorem work for decidability?

Rice's theorem states: Every nontrivial property of recursively enumerable language is undecidable. I came across following problems, which Ullman's books say both are undecidable: Turing ...
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2answers
540 views

Can I apply Rice's theorem to decide decidability status of these languages?

I came across these languages: A Turing machine prints a specific letter. A Turing machine computes the products of two numbers I was guessing whether I can apply Rice's theorem to decide upon above ...
0
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0answers
89 views

Is the language of turing machines, which return epsilon on its own encoding, decidable?

Is the language $\{ \langle M\rangle | f(\langle M\rangle)=\epsilon\}$ decidable? $f()$ means, that the turing machine returns $\epsilon$ on its own encoding and $\langle M\rangle$ stands for the ...
3
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1answer
143 views

Can current quantum computers decide languages that Turing Machines cannot?

I am currently learning Computing Theory at university, and we were on the topic of Turing-Decidability, Recognizability, etc. Showing that a problem is undecidable with Turing machines due to ...
2
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1answer
792 views

How undecidable is it whether a given Turing machine runs in polynomial time?

The proof of Theorem 1 that PTime is not semi-decidable in this recent preprint effectively shows that it is $\mathsf{R}\cup\mathsf{coR}$-hard. The proof itself is similar to undecidability proofs at ...
0
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1answer
440 views

Turing recognizable but not Turing decidable language cannot have TM do not halt on infinitely many inputs

Sorry, I think I misunderstand the question, It should read as if $L$ is turing-recognizable but not decidable, then there exists infinitely many input that any TM will not halt on it...
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0answers
22 views

Is the language $L$=$\{<D_1,D_2> | D_1,D_2$ are DFAs over $\{0,1\}$ and $L(D_1) \subseteq L(D_2)\}$ decidable?

I came up with an algorithm to decide this language, but not sure if it is correct? ...
1
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1answer
749 views

Undecidable: $w$ on which a TM M $M$ halts after $\leq w$ steps

The detailed question is: Is there a word $w$ on which a TM M $M$ halts after a maximum of $|w|$ (word length) steps? I highly assume, that this problem is not decidable because in the worst case ...
0
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1answer
63 views

Undecidability of TMs recognizing a decidable language

The language $L = \{ \text{M} \mid \text{M is a TM and the set of words w such that M halts on w is decidable} \}$ is given. I need to prove that $L$ is NOT Turing recognizable. I've got a hint: it ...
0
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1answer
80 views

Doubt regarding Cantor's diagonalization argument [closed]

So, we use Cantor's diagonalization argument to prove that the Universal Turing Machine is not a decider. I understand the overall argument but have a problem regarding one caveat mentioned in my ...
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2answers
2k views

Proving the decidability of whether a CFG generates a particular string or not

Let $G$ be a context-free grammar and $w$ be a string of length $|w| = n$. Consider the language $A_{CFG}$ = { <$G$, $w$> | $G$ is CFG that generates $w$ }, where <$G$, $w$> is a string ...
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2answers
88 views

Is decidability closed under the mapping f where f(a)=f(b)=0 and f(c)=1?

Consider the function $f$ that maps strings over $\{a, b, c\}$ to strings over $\{0, 1\}$ by replacing each $a$ by 0, each $b$ by 0, and each $c$ by 1. For example $f(cabbc) = 10001$. The function $f$ ...
0
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1answer
488 views

Reducing the halting problem for a language with strings that include at least one 1

$L_1$ = A sequence of $0$ or $1$'s such that at least one $1$ is in the sequence $L_2$ = Turing machines that decide $L_1$ I think the first language is decideable, as the input string is of finite ...
-1
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2answers
198 views

Decidable questions of undecidable problems

Even if there is no general algorithm to decide if any program will halt, but there could be properties or meta-questions about the programs that is decidable. For example, given program $A$ and a ...
0
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1answer
55 views

I want to know where there is the flaw in my argument

I came across following problem to finding whether the following language is decidable or semi-decidable or not even a semi-decidable. $L: \{\langle M\rangle: M\space is\space a\space TM\space and\...
-1
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1answer
341 views

Why is it not possible to prove that two Turing Machines calculate the same function?

I was wondering why it is not possible. Is it because the corresponding language is not decidable, or because of the fact that it is not guaranteed that a Turing machine halts on every input?
1
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1answer
179 views

How can $A \cup B$ be decidable if $B$ is undecidable?

My assignment says: "Determine if the following statement is correct: If $A$ and $A \cup B$ are decidable, then $B$ is decidable." The solution says: "Incorrect. If $B = H_0 \subseteq \{0,1\}^*$ ...
1
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2answers
81 views

What are some decidable problems which cannot be solved in real life(due to time and memory constraints)?

The first line of Sipser book for the Chapter- 'Time complexity', says that: Even when a problem is decidable and thus computationally solvable in principle, it may not be solvable in practice if the ...
4
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1answer
48 views

Language containing all unambiguous grammars

Suppose $L$ is the language of the unambiguous grammars. That is, a sentence $w\in{}L$ if it is a string that describes an unambiguous context-free grammar. Considering that deciding whether a ...
1
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0answers
292 views

Proof by reduction and Turing machines [closed]

This is a practice question I have, but I can't wrap my head around it. ............. Let L = {M | M is a TM that halts with exactly two words on its tape in the form Bw1Bw2B}. B = Blank Position the ...
3
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0answers
61 views

Hamming connectivity of regular languages

Call a language $L$ Hamming connected iff, for every pair of strings $x, y \in L^2$, where $|x|=|y|$, $x$ may be transformed into $y$ by a sequence of single symbol in-place replacements, so that ...
0
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1answer
145 views

Decidability of decision problems

Can somebody give intuition how to answer those questions? From one side I can say that most of them are undecidable because we can reduce the halting problem to them (or halting problem can appear ...
0
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1answer
1k views

Why is a subset of a undecidable language decidable?

I have problems with the understanding why a subset of a undecidable language is decidable. We've proved in the lecture that $HALT$$_T$$_M$$=${$<M,w>$|M is a TM and M halts on input w} is ...
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2answers
98 views

Since the halting problem is undecidable, does that mean that there exists an always undecidable program?

The usual demonstration of the halting problem's undecidability involves positing an adversarial machine (call it $A_0$) that runs the decider machine (call it $D_0$) on itself and performs the ...
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1answer
1k views

Rice Theorem - Problem to understand and apply it

I have struggle to understand the Rice Theorem. My understanding of Rice Theorem: The purpose of this Theorem is to proof that some given language L is undecidable iff the language has a non-trivial ...
2
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2answers
2k views

Undecidable languages

I'm confused on the definition of undecidable languages. Definition: For an undecidable language, there is no Turing Machine which accepts the language and makes a decision for every input string w. ...
1
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1answer
217 views

How to detect infinite loop exist in linear bounded automata (LBA)?

The following theorem from Michael Sipser's book "Introduction to the Theory of Computation" states: $A_{\textrm{LBA}}= \{ \langle M, w \rangle \mid \text{$M$ is an LBA that accepts string $w$} \}$....
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1answer
293 views

Differences between ALLTM and INF

The definitions of ALLTM and INF are as follows: $$\mathrm{ALLTM} = \{ \langle M \rangle \mid \text{ TM $M$ such that $L(M) = \Sigma^*$} \}. $$ $$\mathrm{INF} = \{ \langle M \rangle \mid \text{...
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1answer
20 views

Is finite subset of a set which contains all non regular languages a regular set?

Let A be a set which contains all non-regular languages. Then set B which is finite subset of A. Then will it be regular ? I know that A is not recursive enumerable set (undecidable). So I wonder ...
0
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0answers
39 views

I've proven my language undecidable what is left to prove it Turing equivalent?

Let us say that I have a computation model $A$. Let us also say that I have shown that $A$ can be simulated by a Turing machine. I have not been able to prove that $A$ can simulate a Turing machine. ...
2
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1answer
315 views

Probabilistic halting problem

I'm a physics and math student working through Nielsen & Chuang's text on quantum computation and information. I don't have much experience in CS theory, so some of these exercises are confusing ...
3
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1answer
2k views

Which languages, decided by a turing machine are decidable?

How do I decide if a language is decidable and/or semi-decidable? I have theses languages: a) { < M > | L(M) ⊆ 0*} b) { < M > | L(M) contains at least one word of even length} c) {...
2
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1answer
254 views

To check whether the problem of a particular string being a member of CFG G is decidable or not, why can't we use a PDA? [closed]

Why can't a TM simulate a PDA? Then we can easily construct a PDA P which is made from grammar G. And contruct a TM that simulates P to prove that this problem is decidable.

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