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Questions tagged [decidability]

Use this tag for questions about the existence of an algorithm that can and will return a correct true or false value to a decision problem.

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Model Theoretical Interpretation of the Incompleteness of Number Theory

This question was sparked by this Numberphile video: https://www.youtube.com/watch?v=O4ndIDcDSGc. Near the end, (12:05), he speaks about the Riemann Hypothesis. He describes that if Riemann is shown ...
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Do game theoretic issues make the Monty Hall problem undecidable?

If we consider the Monty Hall problem de novo, it is evident that much depends on the strategy of the host. The wiki article on the problem lists possible host strategies. For example, there is Monty ...
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Why can't the sequent calculus for First-Order Classical Logic be used for proving decidability via Proof-search?

I understand that Turing reduced the halting problem to the satisfiability problem of first-order logic thus proving first-order logic undecidable. However, when thinking about the sequent calculus ...
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Difference of two decidable languages?

I've been learning about TMs in class lately and we talked about the decidability of two languages by union or intersection. I was wondering if you have two decidable languages, L1 and L2, is their ...
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Weakened versions of Word and Isomorphism Problems in group theory

Here are my questions: (Weakened Word Problem) Let $\langle X |R\rangle$ be a finite presentation of a group $G$, and let $w$ be an element of the free group $F(X)$. Does there exist an algorithm (...
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Can we enumerate finite sequences which have no halting continuation?

Note: this is a cross-post from CS.SE, since I haven't gotten an answer there. I am trying to deepen my understanding of the relationship between the Halting Problem and Godel's Completeness Theorem (...
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Symbols of the language vs. Free variables

For some context: I'm currently taking a course of Formal Methods and Logics and there's a passage where we show that the monadic second order ($\text{MSO}$) theory of (possibly labelled) linear ...
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Specify a decisive Turing machine that calculates the following function $f$.

Specify a decisive Turing machine that calculates the following function $f$: $$\small f:\{a,b\}^*\to\{a,b\}^*\textrm{ with } f(w)= \begin{cases} (bba)^{3\cdot\#_b(w)}& \text{if } \#_a(w) \text{ ...
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What is the computational complexity of Tarski's arithmetic?

Tarski proved that the first-order theory of real-closed fields is decidable. Is the exact computational complexity known? The best upper bound I could find is EXPSPACE [1], where it is also ...
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Why does the unsolvability of the Halting Problem give a negative solution to the decision problem?

So Hilbert famously asked for a formalization of all of mathematics which was computationally decidable. Gödel is credited with shattering the idea that "all of mathematics" can be formalized. After ...
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How Wang's conjecture implies decidability of The Domino Problem?

Wang stated following conjecture about Wang tiles (which was proven false by R. Berger): A finite set of plates [Wang's tiles] is solvable if and only if it has at least one periodic solution. ...
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does the language 𝐿 = {< 𝑀1 >, < 𝑀2 >: 𝐿(𝑀1 ) ⊆ 𝐿(𝑀2)} is in co-RE?

i was asked to determine if its in RE and if its in co-RE. well i think its easy to say the language is not in RE but i was wondering if this language is in co-RE. so the question is if $\overline{L}$...
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proof that specific disjoint sets are recursively enumerable, but don't lie in a decidable set and its complement

let's call a set $A \subseteq \mathbb{N}$ recursively enumerable if it's "partial characteristic function" $\tilde{\chi}_A$ is computable, whereby $\tilde{\chi}_A$ is defined as: $\tilde{\chi}_A$:= 1, ...
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Decidability of a relation on Functional space!

Suppose I have this functional space $(D=\{ a\searrow b; a \in A , b \in B\}, \leqslant)$ (partial order relation on step functions!),also suppose that relation $\leqslant_1$ is decidable on $(A,\...
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If a language is NOT partially decidable, is the complement not partially decidable?

I am trying to figure out if L is partially decidable or not partially decidable. Let L be {encode(x): x is a Turing machine that halts on input encode(x)}.
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Undecidability of: $|w \in L| \geq 1, L=\{w \in \{0,1\}^*|a_0·\#_0(w)+a_1·\#_1(w)- a_1a_0=0\}$

Let $a_0, a_1 \in \mathbb{N} \setminus \{0\}$ and $L=\{w \in \{0,1\}^*|a_0·\#_0(w)+a_1·\#_1(w)- a_1a_0=0\}$ . Let's assume problem $P$ that, language of Turing machine accepts at least one word from ...
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Example of incomplete, but decidable theory, and of complete and undecidable theory, question

On wikipedia it is written that Decidability should not be confused with completeness. For example, the theory of algebraically closed fields is decidable but incomplete, whereas the set of all ...
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Proving the diagonalization of $U \subset \mathbb{N} \times \mathbb{N}$ that is an universal set for all enumerable sets of naturals is undecidable

Let $U \subset \mathbb{N} \times \mathbb{N}$, and let $U_n = \{ x \mid (n, x) \in U \}$. Let's call $U$ universal for some class $\mathcal{S}$ of subsets of $\mathbb{N}$ if $S \in \mathcal{S} \iff \...
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Gödel's completeness theorem and the undecidability of first-order logic

I'm working through this module, "Undecidability of First-Order Logic" and would love to talk about the two exercises given immediately after the statement of Godel's completeness theorem. First, ...
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Recursion Theory/Incompleteness Theorems: Computability of sets of formulas in first order logic

I am struggling with the following two problems: Suppose that $M$ is a structure with finite universe and finite alphabet. Show that the set of formulas $\{\varphi$ $\mid$ for every $M$-assignment $\...
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The (un)decidability of the Tits Alternative for any given (suitably defined) set of groups.

Please forgive me if this question is ill-formed. I don't know much about decidability. Some Background: There are problems in combinatorial group theory that are undecidable, such as the word ...
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Decidability of $\forall\exists$ diophantine equations

By saying $\forall\exists$ diophantine equations I mean sentences of the form: $\forall x\exists y\,[p(x,y) = 0]$ where $p$ is a polynomial on $x,y$, and both $x,y$ range over natural numbers. I want ...
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Why is ZFC recursively axiomatizable?

I have read that ZFC is recursively axiomatizable, and hence is incomplete by Gödel's theorem. Now why is this true? Consider in particular the axioms of replacement and separation. My guess is that ...
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Halting problem proofs and reduction

Okay so Ive got a problem with the "direction" of reduction and untimately the whole proof, reduction idea as a whole... My Question is: Prove that the problem of deciding whether a Turing Machine ...
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$L_1= \{\langle M\rangle \mid $ there exists $x \in \Sigma^*$ such that for every $y \in L(M), xy \notin L(M)\}.$ Is $L_1$ RE or not RE?

I tried to prove $L_1$ is not Recursively enumerable via Rice's theorem, however i've been told by a mentor that examples i used are not valid. Can someone point out the mistakes in my understanding? ...
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Proof in constructive mathematics using decidability.

I am working in constructive mathematics that means without the law of excluded middle. One may also interpret this as working in inuitionistic logic. Lets assume I have some set $A$ such that I ...
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Decidability context-sensitiv and context-free grammars

Show that it is unsolvable whether a given context-sensitive language is context fre. And, show that the emptiness problem is solvable for one-way nondeterministic stack automata. I don't know how ...
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Are soundness and completeness necessary for decidability? [closed]

Is it necessary for a logical system to be both sound and complete in order to be decidable?
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Why is the set of all tautologies in propositional logic recursively enumerable?

I've just started reading an english edition of Gems of Theoretical Computer Science. On page 5 (or page 12 of the overall pdf), the author brings up the set of all tautologies in propositional logic ...
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Large Cardinals and Diophantine Equations (Penelope Maddy)

Professor Penelope Maddy remarks without elaboration in her famous 'Believing the Axioms' essay that 'It should be mentioned that the Axiom of Inaccessibles also has a few extrinsic merits. It ...
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Algorithm to decide the universality / functional completeness of a set of logic gates?

Given a set of logic gates $G$, let $F_G$ denote the set of all formulas composed of gates from $G$. We say that $G$ is "universal for computation" or "functionally complete" if it forms a basis for ...
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Converting formula to a closed form with only existential quantifiers

Let there be some formula $\phi$, is there an algorithm to construct a closed formula $\phi'=\exists x_1...\exists x_n \psi$ where $\psi$ does not have any quantifiers and $\phi$ is satiable iff $\phi'...
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Prove the theory of equality with a finite number of unary predicates is decidable

Let the signature have $n$ unary predicate symbols $P_1, \dots, P_n$ and a single binary predicate $=$. Consider the theory of equality with the following axioms: $\forall x (x = x)$ $\forall x \...
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Is there a semi-decidable statement equivalent to the Collatz-conjecture?

We cannot rule out that the Collatz-conjecture cannot be proven. But we also cannot rule out that it is false and we cannot prove this in the case the sequence diverges for some start-number. Is ...
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Is any context-free grammar in Chomsky form with at most 2017 rules necessarily finite?

A task from one of the former tests: Is there any algorithm, which for all pairs of context-free grammars over $\lbrace a,b,c\rbrace$ in Chomsky form $G_1,G_2$ with at most $2017$ rules correctly ...
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1answer
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A axiomatization of (full) Second Order Logic with a decidable proof system cannot be complete; is this true if we only require semi-decidability?

My understanding is that, unlike first order logic, no "effective" (sound, consistent) axiomatization of second order logic is complete; there will always be statements true in all models, but not ...
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Choosing axiom schemes for a logical theory

In a Hilbert system, there are many ways that we can choose axiom schemes. My question is: 1- How do we know that we have defined enough schemes? What would happen if I remove a scheme from the list? ...
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Is the semidecidability of the valid formula of second order logic dependent upon the semantic?

This is perhaps a stupid question, but I ask it anyway. It seems to me that the semantic comes after and it cannot change the complexity of the language. I ask the question, because Herbert B. ...
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Prove whether the problem is decidable [closed]

Main topic is to decide if the problem that selected partial μ-recursive function is an injective function is decidable, undecidable or semi-decidable. The function is injective when $\forall a,b\;f(...
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Decidability if a given expression is equal to a prime number

Let us assume there is a number which is well-defined and computable, but it is hard to compute it. E.g., $x=\pi^{(\pi^{(\pi^\pi)})}$. It is not even known if the given number is an integer (which is ...
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Is it semi-decidable whether a context-free grammar generates a regular language?

It is a well-known that it is undecidable in general whether an arbitrary context-free grammar generates a regular language. However, I could not find any results concerning the question whether this ...
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Does double negation elimination hold for decidable formulae in intuitionistic logic?

Let $P$ be a quantifier-free decidable formula, i.e. one can prove $P \lor \lnot P$. Does it follow that $\lnot \lnot P \to P$ intuitionistically? Informally, a decidability of a formula means ...
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Does solving the complexity class ALL collapse all Turing degrees?

I came across this paper by Scott Aaronson and though I understand nothing of quantum computing, the fact that there was an (even hypothetical and probably unrealizable) model of ...
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Do given Turing Machines M,N accepts equinumerous languages?

I was doing some exercises from computability and complexity and then I have stuck on this problem: What type of problem (decidable, semi-decidable, undecidable) is the problem (show it): Do given ...
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Why is this TM problem decidable

Please explain to me (like I'm an idiot) how the problem Does M on input Λ ever write a nonblank symbol on the tape? given TM M as an input is decidable. To ...
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Application of Rice's Theorem

How can I prove, by applying Rice's theorem, that the language L is undecidable? $L = \lbrace \alpha : M_{\alpha}(x) =x^2 \,\,\, \forall x \in \lbrace 0,1\rbrace^* \rbrace $ I think this is a ...
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Prove that every infinite regular language has an undecidable infinite subset

I am having trouble writing a formal proof for this. I understand that we have an infinite regular language. This means that we have uncountable many subsets of the infinite regular language and due ...
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Can the question whether $x^a+y^b+z^c=n$ has a solution over the integers be undecidable?

Suppose, $a,b,c \ge 1$ are integers. Can the question whether the equation $$x^a+y^b+z^c=n$$ has a solution in integers $x,y,z$ for some particular integer $n$ be undecidable ? I ask because I ...
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Is First Order Logic + Arithmetic semi-decidable?

I understand that it is not complete, but is it decidable, semidecidable or not decidable? Also, does something have to be complete for it to be considered decidable or semidecidable? Meaning, can ...
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Classify language as decidable, undecidable but recognisable or unrecognizable

I'm currently studying unrecognizable languages in Turing Machines and came across this problem L1 := {< M > | M is a TM and M accepts at least one string w in {0,1}* with more zeros than ones} I ...