Questions tagged [computability]

Questions related to computability theory, a.k.a. recursion theory

40
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4answers
11k views

What are common techniques for reducing problems to each other?

In computability and complexity theory (and maybe other fields), reductions are ubiquitous. There are many kinds, but the principle remains the same: show that one problem $L_1$ is at least as hard as ...
43
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2answers
13k views

How to show that a function is not computable?

I know that there exist a Turing Machine, if a function is computable. Then how to show that the function is not computable or there aren't any Turing Machine for that. Is there anything like a ...
130
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3answers
14k views

How can it be decidable whether $\pi$ has some sequence of digits?

We were given the following exercise. Let $\qquad \displaystyle f(n) = \begin{cases} 1 & 0^n \text{ occurs in the decimal representation of } \pi \\ 0 & \text{else}\end{cases}$ ...
53
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6answers
11k views

Are there minimum criteria for a programming language being Turing complete?

Does there exist a set of programming language constructs in a programming language in order for it to be considered Turing Complete? From what I can tell from wikipedia, the language needs to ...
29
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2answers
4k views

Why are the total functions not enumerable?

We learned about the concept of enumerations of functions. In practice, they correspond to programming languages. In a passing remark, the professor mentioned that the class of all total functions (i....
141
votes
11answers
48k views

Why, really, is the Halting Problem so important?

I don't understand why the Halting Problem is so often used to dismiss the possibility of determining whether a program halts. The Wikipedia article correctly explains that a deterministic machine ...
36
votes
2answers
7k views

Perplexed by Rice's theorem

Summary: According to Rice's theorem, everything is impossible. And yet, I do this supposedly impossible stuff all the time! Of course, Rice's theorem doesn't simply say "everything is impossible". ...
27
votes
7answers
6k views

What are the simplest examples of programs that we do not know whether they terminate?

The halting problem states there is no algorithm that will determine if a given program halts. As a consequence, there should be programs about which we can not tell whether they terminate or not. ...
14
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3answers
1k views

What are the possible sets of word lengths in a regular language?

Given a language $L$, define the length set of $L$ as the set of lengths of words in $L$: $$\mathrm{LS}(L) = \{|u| \mid u \in L \}$$ Which sets of integers can be the length set of a regular language?...
19
votes
1answer
944 views

Do fully optimizing compilers for terminating programs exist?

In Andrew W. Appel's book, Modern Compiler Implementation in ML, he says under chapter 17 that Computability theory shows that it will always be possible to invent new optimizing transformations and ...
58
votes
10answers
9k views

Human computing power: Can humans decide the halting problem on Turing Machines?

We know the halting problem (on Turing Machines) is undecidable for Turing Machines. Is there some research into how well the human mind can deal with this problem, possibly aided by Turing Machines ...
38
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9answers
8k views

Is C actually Turing-complete?

I was trying to explain to someone that C is Turing-complete, and realized that I don't actually know if it is, indeed, technically Turing-complete. (C as in the abstract semantics, not as in an ...
37
votes
1answer
26k views

Is a push-down automaton with two stacks equivalent to a turing machine?

In this answer it is mentioned A regular language can be recognized by a finite automaton. A context-free language requires a stack, and a context sensitive language requires two stacks (which is ...
30
votes
2answers
4k views

NP-Hard problems that are not in NP but decidable

I'm wondering if there is a good example for an easy to understand NP-Hard problem that is not NP-Complete and not undecidable? For example, the halting problem is NP-Hard, not NP-Complete, but is ...
22
votes
6answers
3k views

Algorithm to solve Turing's “Halting problem‍​”

"Alan Turing proved in 1936 that a general algorithm to solve the halting problem for all possible program-input pairs cannot exist" Can I find a general algorithm to solve the halting problem for ...
11
votes
2answers
5k views

Single-tape Turing Machines with write-protected input recognize only Regular Languages

Here is the problem: Prove that single-tape Turing Machines that cannot write on the portion of the tape containing the input string recognize only regular languages. My idea is to prove that this ...
29
votes
7answers
5k views

Is there a more intuitive proof of the halting problem's undecidability than diagonalization?

I understand the proof of the undecidability of the halting problem (given for example in Papadimitriou's textbook), based on diagonalization. While the proof is convincing (I understand each step of ...
17
votes
2answers
6k views

How is the rule 110 Turing complete?

I've read the wikipedia page for rule 110 in cellular automata, and I more or less know how they work (a set of rules decides where to draw the next 1 or 0). I've just read they're Turing complete, ...
11
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3answers
618 views

References on comparison between quantum computers and Turing machines

I was told that quantum computers are not computationally more powerful than Turing machines. Could someone kindly help in giving some literature references explaining that fact?
18
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2answers
10k views

Is the set of Turing machines which stops in at most 50 steps on all inputs, decidable?

Let $F = \{⟨M⟩:\text{M is a TM which stops for every input in at most 50 steps}\}$. I need to decide whether F is decidable or recursively enumerable. I think it's decidable, but I don't know how to ...
29
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1answer
2k views

Does there exist a Turing complete typed lambda calculus?

Do there exist any Turing complete typed lambda calculi? If so, what are a few examples?
15
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2answers
3k views

Which languages are recognized by one-counter machines?

Counter machines with two or more counters are typically shown to be equivalent to Turing machines in courses on the theory of computation. However, I have not seen a formal analysis of which ...
11
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6answers
22k views

Can a Turing machine decide the language $L_\emptyset$ of machines with empty language?

Let $$L_\emptyset = \{\langle M\rangle \mid M \text{ is a Turing Machine and }L(M)=\emptyset\}.$$ Is there a Turing machine R that decides (I don't mean recognizes) the language $L_\emptyset$? It ...
7
votes
1answer
3k views

Why can't we solve the Halting Problem by using Artificial Intelligence? [duplicate]

Yesterday I was reading about Computability and they mention the Halting Problem. It got stuck in mind all day until I remember that some weeks ago, when learning Java, the IDE (Netbeans) show me a ...
10
votes
1answer
901 views

Are there any existing problems that wouldn't be solvable with a halting oracle?

I understand that most problems are trivial if a halting oracle is available (or, I think equivalently, hyper-computation). However, applying the argument that shows the Halting Problem is impossible ...
8
votes
3answers
452 views

Can the encodings set of a non-trivial class of languages which contains the empty set be recursively enumerable?

Let $C$ be a non-trivial set of recursively enumerable languages ($\emptyset \subsetneq C \subsetneq \mathrm{RE}$) and let $L$ be the set of encodings of Turing machines that recognize some language ...
15
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3answers
499 views

Turing complete and computational power

In a lecture a professor mentioned that modern computers don't have as much computational power as a Turing machine because they don't have infinite memory, and since no computer can have infinite ...
13
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2answers
1k views

Decidable restrictions of the Post Correspondence Problem

The Post Correspondence Problem (PCP) is undecidable. The bounded version of the PCP is $\mathrm{NP}$-complete and the marked version of the PCP (the words of one of the two lists are required to ...
3
votes
1answer
834 views

Show that Halting problem $(\mathsf{HP\text{}})$ is $\mathsf{NP\text{-}Hard}$

Let me define first Halting problem $(\mathsf{HP\text{}})$. Given : $(M , x)$, $M$ is a turing machine and $x$ is a input binary string to turing machine $M$. Decide : Does $M$ halt on string $x$?...
2
votes
1answer
1k views

Is regularity of the language accepted by a given Turing machine a semi-decidable property?

Given is the definition of a general problem: $\{ \langle M, S\rangle \mid M \text{ is a } TM, L_M \in S\}$. In words: Given a TM M, does M decide a language that is an element of the given set of ...
18
votes
5answers
878 views

Is it possible to solve the halting problem if you have a constrained or a predictable input?

The halting problem cannot be solved in the general case. It is possible to come up with defined rules that restrict allowed inputs and can the halting problem be solved for that special case? For ...
11
votes
1answer
658 views

Reductions among Undecidable Problems

Im sorry if this question has some trivial answer which I am missing. Whenever I study some problem which has been proven undecidable, I observe that the proof relies on a reduction to another problem ...
3
votes
1answer
1k views

Can a recursive language be uncountable?

Does there exist a recursive language $L$ whose cardinality is uncountable? I would like to have an explanation whether Turing Machine can encode uncountable languages and whether we can use this to ...
9
votes
3answers
830 views

Decidable languages and unrestricted grammars?

Turing machines and unrestricted grammars are two different formalisms that define the RE languages. Some RE languages are decidable, but not all are. We can define the decidable languages with ...
28
votes
2answers
5k views

Why are there more non-computable functions than computable ones?

I'm currently reading a book in algorithms and complexity. At the moment I'm, reading about computable and non-computable functions, and my book states that there are many more functions that are non-...
31
votes
5answers
7k views

Proof that dead code cannot be detected by compilers

I'm planning to teach a winter course on a varying number of topics, one of which is going to be compilers. Now, I came across this problem while thinking of assignments to give throughout the quarter,...
8
votes
4answers
7k views

Does our PC work as Turing Machine?

Does our PC work as Turing Machine? The model of a Turing Machine consists of infinite memory tape, which means infinite states. But suppose if our PC has 128 MB memory and 30GB disk it would have ...
29
votes
1answer
2k views

Rice's theorem for non-semantic properties

Rice's theorem tell us that the only semantic properties of Turing Machines (i.e. the properties of the function computed by the machine) that we can decide are the two trivial properties (i.e. always ...
4
votes
2answers
4k views

What's a trivial property?

I have to show a property P is trivial. This problem has to do with Rice's Theorem, which I do not completely understand. Can someone explain the difference between trivial and non-trivial properties?...
9
votes
2answers
3k views

Infinite alphabet Turing Machine

Is a Turing Machine that is allowed to read and write symbols from an infinite alphabet more powerful than a regular TM (that is the only difference, the machine still has a finite number of states)? ...
6
votes
1answer
5k views

Relationship between Undecidable Problems and Recursively Enumerable languages

I have read the Wikipedia article on Recursively Enumerable languages. The article suggests that the halting problem is recursively enumerable but undecidable. My idea till today was that the halting ...
6
votes
2answers
252 views

Is there an always-halting, limited model of computation accepting $R$ but not $RE$?

So, I know that the halting problem is undecidable for Turing machines. The trick is that TMs can decide recursive languages, and can accept Recursively Enumerable (RE) languages. I'm wondering, is ...
3
votes
1answer
6k views

Is every subset of a decidable set, also decidable?

Is it true that if A is a subset of B, and B is decidable, than A is guaranteed to be decidable? I believe it would be true because all the subsets of B should also be decidable making A decidable. I'...
3
votes
1answer
288 views

Construction of the complement of universal Turing machine - where is the catch?

This is pretty fundamental but I'm getting confused. Let $U$ be the Universal Turing Machine and $L_{u}$ the language it accepts which is recursively enumerable. Obviously we are not able to construct ...
2
votes
2answers
4k views

Is the language TMs that accept finite languages Turing-recognizable?

I know that $L=\{ \langle M \rangle \mid |L(M)| < \infty \}$ is not decidable (by Rice's theorem or using reduction, I followed it from $L$ not being decidable ). But is $L$ recognizable? What I ...
1
vote
1answer
2k views

Right moving turing machine

I am interested in simulating any turing machine with a turing machine that is allowed only to move right. I guess that it should be pretty standard material and likely it is trivial (or known to be ...
16
votes
2answers
2k views

Decidablity of Languages of Grammars and Automata

Note this is a question related to study in a CS course at a university, it is NOT homework and can be found here under Fall 2011 exam2. Here are the two questions I'm looking at from a past exam. ...
3
votes
3answers
2k views

Proof that whether a regular language is finite is decidable [duplicate]

I have this question for a homework. The question stems from the fact that you can determine whether a regular language is empty by using a Turing machine to count the states n in the given FSM. When ...
3
votes
1answer
821 views

Language consisting of all Turing machine encodings [closed]

$A=${$ ⟨M⟩$:$M$ $is$ $a$ $Turing$ $Machine$ } What can be said about $A$ ? Specifically, is $A$ decidable,regular,CFL,CSL? I would say $A$ is decidable since we can write an algorithm to check ...
2
votes
1answer
116 views

Which one of these two sets is computably enumerable?

M is a turing machine description, L(M) is recognized by M, |L(M)| is the size of this language. {M : |L(M)| <= 330} {M : |L(M)| >= 330} I don't quite understand what this question is asking. ...