Turing machines and decidability

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Discussion Overview

The discussion revolves around the decidability of whether a Turing machine can reach a specific configuration with a given state. Participants explore the implications of this problem in relation to the halting problem and the nature of decidability in computational theory.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether a configuration αsβ can yield a configuration with state q and seeks an algorithm to determine this.
  • Another participant asserts that the problem is harder than the halting problem, which is known to be undecidable.
  • A request for a detailed explanation of why the problem is undecidable is made, indicating a desire for deeper understanding beyond comparisons to the halting problem.
  • A participant proposes that given an initial state s, it might be possible to decide if there exists a configuration leading to state q, suggesting that the problem could be decidable.
  • Another participant argues that if an algorithm could decide this problem, it could be used to solve the halting problem, thus concluding that no such algorithm can exist and reinforcing the undecidability of the original problem.
  • A later reply expresses clarity on the topic after the discussion, indicating that the points raised were helpful.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the decidability of the problem. There are competing views, with some arguing for decidability and others asserting its undecidability based on its relationship to the halting problem.

Contextual Notes

The discussion highlights the complexity of the problem and the reliance on the definitions of configurations and states within Turing machines. The relationship between the proposed problem and the halting problem remains a critical point of contention.

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Okay, so if I have a turing machine so called M, is there a configuration alpha s beta which yields a configuration with state q? If it's decidable can someone explain the algorithm to proceed?
 
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That seems to be clearly harder than the halting problem, which is undecidable.
 
can you provide me with more detailed explanation why this is undecideable? I mean more saying than it's harder than the halting problem, as I can see that as well

i've also scanned the question directly from the book in case it helps
 

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and here's my answer so far:

Given the initial state s we can decide whether there exists a configuration αsβ and reading all the instructions of the k-cells of the TM machine that there is a path from that configuration to γqη where q is a given state of M and αβγη εΓ because there must exist a path according to the given instructions to reach from state s to q with the given configuration.

So I'd say it's decideable.. please correct me if I am wrong
 
So let's say you produce an algorithm that decides this problem.

I can then feed into your algorithm any turing machine M I like, and along with it feed in state q where I choose q to be the halting state for M. If your algorithm works, then your algorithm will tell me whether or not q is reached, and therefore it will have told me whether M halts. But that would mean you have solved the halting problem, therefore there does not exist any such algorithm.

This is what CRGreathouse means by your problem being "harder than the halting problem", it reduces to the halting problem. If you had an algorithm for deciding this problem then you could use the same algorithm to decide the halting problem. This (reducing the problem given to the halting problem) is generally the standard method for proving undecidability...
 
ok now it's clear! thanks guys
 

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