Questions tagged [computability]
Computability theory a.k.a. recursion theory.
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Many-one equivalence of sets that differ finitely
[This is a duplicate of my question from Mathematics Stack Exchange:
https://math.stackexchange.com/questions/4792354/many-one-equivalence-of-sets-that-differ-finitely
I am posting it here since it ...
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Technical limitations of Turing machines due to the input and output encoding of values
Convention: Since I will be asking about some technicalities around Turing machines, it behooves to give a precise definition: say, here, “Turing machine” will stand for a $2$-symbol $1$-tape machine ...
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Computability of Time Complexity of Recursive Sets
Is every recursive set's worst-case time complexity a total recursive function?
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Computational power of probabilistic automata
I am a bit confused about the proper role of probabilistic automata (PA) in the theory of computation.
Informally, I can imagine they can accept more than finite automata (FA) as they, for instance, ...
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Many-one degrees of some particular sets
Let $W_0, W_1, W_2,\dotsc$ be an effective numbering of r.e. sets.
Consider sets $\text{Emp}=\{x\mid W_x=\emptyset\}$, $\text{Tot}=\{x\mid W_x=\mathbb{N}\}$ and $S_n=\{x\mid W_x=W_n\}$ (for some fixed ...
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Decidability of the complexity of decision problems
This might be a question that is related to some of the existent questions on the topic in the title, but I still find some answers either not full, or the topic still slightly different (maybe due to ...
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Self Referential Undecidability Construed as Incorrect Questions
Please see my answer before you read any of this. The answer says the same thing much much more clearly
PhD computer science professor Rick Hehner and I independently derived what we mutually agree ...
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Primitive recursion with varying parameters
Suppose $g\colon \mathbb{N}^k \to \mathbb{N}$, $v_1,\ldots,v_r\colon \mathbb{N}^k \to \mathbb{N}^k$ and $h\colon \mathbb{N}^{k+r+1} \to \mathbb{N}$ are all primitive recursive, and define $f\colon \...
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Complexity of analytic functions and integrals
There exist polynomial - time computable functions, log - space computable functions, and NC - functions. Given this:
To which class do analytic elementary functions, including trigonometric ones, ...
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Primitive recursion relative to a logical system
In various places I have read that the normally considered non-primitive recursive Ackermann function is primitive recursive in higher-order logic. It's claimed to be due to "Reynolds, 1985",...
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Complexity measures for semi-decidable problems
Is there any sensible complexity measure that makes sense to compare the "hardness" of undecidable semi-decidable problems? Time and space are of course not suitable, because they cannot be ...
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What is the meaning of the additive epsilon term in the definition of a time constructible function?
There is a well-known theorem that states that a function $f$ is time constructible if and only if $f$ can be computed in time $O(f)$. But this theorem comes with some conditions: $f$ must be a ...
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Is P=NP relative to the halting oracle?
Consider the following variant $\mathscr{H}$ of the halting oracle: given the code $e$ for an ordinary Turing machine and an input $n$ to it, we let $\mathscr{H}(\langle e,n\rangle) = \langle 0,0\...
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Is there a well-defined notion of an “R/poly” complexity class?
This would be the complexity class of all problems that are decidable in finite time with a polynomial length advice string that can be arbitrarily hard to compute. But potentially undecidable without ...
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What is formal definition of non-deterministic algorithm in context of primitive/general recursion?
I want to understand general method for formally defining non-deterministic algorithm. But all formal definitions I see are related to FSM/Turing-machines.
What is the reference for non-deterministic ...
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Do realizable systems always have some non-well-founded sets?
Suppose we are standing outside a realizable system which admits CZF or a similar constructive set theory. Then consider the following:
LEM is not realized (e.g. this MSE answer)
The traditional ...
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Generalizing Quines: Outputting an Arbitrary Function of Source Code
A Quine is a (non-empty) program $P$ that takes no inputs and returns its own source code $\langle P\rangle$ as the only output. For a function $f$ (with appropriate domains and range) define an $f$-...
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How to prove that a language that allows infinite loops is still not Turing-complete?
CPS translations will always use pairs, either explictly or by currying. Though I can't find a reference for that, I'm assuming this is a necessary condition (I'd appreciate a reference if someone has ...
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Input-Output Machines
From what I know, there is a vast literature on language recognizers in computer science.
Language recognizers are machines (e.g., Finite State Automata, Pushdown Automata, Turing Machines, ...) that, ...
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Reference for "A Turing machine cannot be equipped with an oracle for itself"
Starting with a basic Turing machine, successive applications of the Turing jump produce successively more powerful machines, that can be indexed by any ordinal. A tempting error for students, and one ...
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Computability for universal quantum turing machines
I would like to ask if anyone has any ideas about what a universal quantum turing machine (UQTM) can do as supposed to a classical universal turing machine (UTM) (i.e. quantum computer vs classical ...
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Does Bellantoni-Cook safe recursion (or any other implicit characterization of P) admit Kleene's second recursion theorem?
Abstractly, by a programming language that operates on binary strings I mean a set $P$ of programs along with a semantics relation $[p](x) = y$, ``the program $p$ on string $x$ halts with output $y$.&...
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Extension of primitive recursion, that is as powerful as System-T
I know that System-T restricted to first-order types is exactly as powerful as primitive recursive functions, because I proved it in Agda.
I asked myself, if there is a extension of primitive ...
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Can you see that the Linz Halting Problem proof contains a fatal flaw?
Applying a Simulating Halt Decider to the Linz Halting Problem Proof
When a simulating halt decider correctly simulates N steps of its input it derives the exact same N steps that a pure UTM would ...
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Are there parsers that can parse languages in more expressive grammars than context-free grammars?
The Earley Parser is able to parse all context-free languages. Are there parsers that can parse say languages in context-sensitive grammars? I realize ambiguous grammars are non-deterministic and ...
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Gurevich's theorem on primitive recursive functions being logspace-computable
I recently came across the following result attributed to Gurevich, according to which I understood that the class of problems solvable by primitive recursive functions is precisely the class L of ...
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Generalization of computability to continuous for loops? [closed]
A computable function, formulated in the sense of mu recursion, can compute a for or do loop over some (possibly infinite) integer range.
I was wondering if a suitable generalization exists that ...
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An analogue of Scott continuity for infinite-time-Turing-computable functions
$\newcommand{\tb}{2_{\!\bot}}\newcommand{\tbO}{\tb^{\,\omega}}$Let $2 = \{0,1\}$ and $\tb = \{0,\bot,1\}$.
Scott continuity is important for defining models of lambda calculus,
a formalism for ...
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Are there more learnable but undecidable cases except the halting problem
Per request, I cross post the question here which is original from math.stackexchange
In the ICML 1996 paper, On the Learnability of the Uncomputables, by Richard Lathrop, he proved that halting ...
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What are the application of Scott-Topology in theoretical computer science?
During a work I came across the Scott-Topology and I see that Scott-continuous functions show up in the study of models for lambda calculi. What I cannot understand is how this enrich the lambda-...
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Understanding the construction of an uncomputable function
The following is from Arora and Barak's "Computational Complexity." I think one does not have to read the second paragraph of the proof to answer this question.
Theorem 1.10 There exists a ...
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How can I switch into computability theory when I am part way through my PhD in deep learning?
I am partway through my PhD studying deep learning. I chose it just because it's useful and would yield a lot of industry opportunities. However, I am really missing my previous coursework in ...
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Expressive power of lambda-calculus with restricted application
Consider a syntactic restriction of the (untyped) $\lambda$-calculus in which an application cannot have another application as an immediate subterm. More precisely, restricted terms ($R,S,...$) and ...
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Why are some classes (ALL, ELEMENTARY, R, etc) badly behaved as oracles?
Some classes, such as ALL, ELEMENTARY, and R, are very badly behaved when used as oracles. For instance, all three of these classes trivially collapse P and EXP, even though (by the Time Hierarchy ...
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Detailed proof of Theorem 2.1 in Papadimitrou book (Multitape TM to SingleTape TM)
I want to know if anybody knows a detailed proof of Theorem 2.1 of Papadimitrou's book Computational Complexity. The theorem states "Given any $k$-string Turing machine $M$ operating within time $...
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Halting problem for finitary PCF
Is the halting problem decidable for finitary PCF? By "halting problem" I mean the problem of deciding whether a closed PCF term evaluates to bottom under the denotational semantics of PCF. ...
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Is there a fundamental link between Nash's equilibrium and Turing's halting problem?
Since Nash equilibrium exists, is there a computational analogue of this equilibrium point? I am trying to approach Nash equilibrium from computational point of view to see if the equilibrium point ...
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What is a known sequence for which being constant is not provable?
My question concerns the property of being constant for computable functions ${\mathbb N}\to \{0,1\}$, within any common framework $T$ strong enough to include Heyting arithmetic (and of course not ...
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What is the complexity class of higher-order primitive recursion?
Short and sweet: The complexity class of primitive recursive functions (WP) is PR (WP, Zoo). What's the complexity class of higher-order primitive recursion (WP)?
The motivating context is simply that ...
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A variant of two-counter machine
I would like to show that the halting problem for some variant of two counter machine (Minsky machine) is undecidable:
instead of "if c=0 goto i else goto j", there are "if c>d goto ...
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Is coRE closed under concatenation?
I know that RE is closed under union, intersection, and concatenation (but not complement). It is likewise easy to show that coRE is closed under union and intersection (but not complement). What ...
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Is BigInteger-based Brainfuck Turing Complete?
All of the proofs of Turing-Completeness I've found for Brainfuck rely on its cells being fixed-width integers that wrap around upon over/underflow. The "parent language" P'' on which ...
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Halting behavior of a randomly selected Turing machine?
Let $TM(k,2)$ be the set of Turing machines with $k$-states and $2$ symbols. Let $h(k)$ be the number of machines in $TM(k,2)$ that halt when run on the blank input.
Is
$\lim_{k \to \infty} \frac{h(k)...
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Is relativization statement-dependent or proof-dependent?
I'm relearning some computability theory, and have encountered the idea of relativization of results to arbitrary subsets of $\omega$ and the subtlety of figuring out what the correct relativized ...
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Where, if any, is there currently any research being done on the subject of ternary computers? [closed]
I had the experience several years ago of working with a team that had developed a ternary computing system. It ran out of funding and was abandoned but I felt it was ahead of its time. Currently, ...
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Proof and computational complexity
I couldn't find documents elaborating on this: if the Curry Howard correspondence is to be interpreted as establishing a strong relation between proofs and programs, should there not be a strong ...
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A proved computationally-irreductible function
In arXiv:1111.4121 and arXiv:1304.5247, Hervé Zwirn and Jean-Paul Delahaye propose a formal definition of computational irreducibility. Is there a (possibly artificial) function that can be proved to ...
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What is the difference between a model of computation and a programming language?
https://en.wikipedia.org/wiki/Model_of_computation includes sequential models, functional models and concurrency models.
Sequential models include finite state machine, Turing machines, random access ...
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Is this variant of bitwise cyclic tag Turing-complete?
Cross-posted to MathOverflow.
CT is an extremely minimalist programming language that can simulate arbitrary tag systems, and is therefore Turing-complete. A program consists simply of a string of 3 ...
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What is the meaning of 'uniform' in Blum's paper "On the Size of Machines"
In Blum's "On the Size of Machines" he uses the word 'uniform' in several places.
For instance, in Theorem 1, he writes
Let $f$ be any recursive function. Then there exists $i,j\in \mathbb{...
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