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I want to solve the following exercise in Computability and Complexity Theory:

By providing a reduction from the HALTING problem to REACHABLE-CODE, prove that REACHABLE-CODE is undecidable.

REACHABLE-CODE is defined like this:

INSTANCE: A source code S, a number n of a line in S. QUESTION: Is there an input I for S such that the run of S on I will reach the code on line n?

I am not sure how to solve this. Can i Solve this by assuming that ther IS an algorithm for the REACHABLE-CODE problem and use it as a subroutine in an alrogithm that solves the HALTING problem?

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    $\begingroup$ Why do people keep tagging questions about deciability with [decision-theory]? $\endgroup$ – Asaf Karagila Apr 12 '13 at 15:36
  • $\begingroup$ @AsafKaragila: My guess: because there isn't a "decidability" tag. If you start typing "deci", the only options are "decimal-expansion" and "decision-theory", so for someone who doesn't what decision theory is, the latter seems closer to "theory of decidability". (But I guess you knew this already, and your question wasn't intended to be taken literally.) $\endgroup$ – ShreevatsaR Apr 12 '13 at 15:39
  • $\begingroup$ @ShreevatsaR: Obviously my question was meant as a rhetorical rant... ;-) $\endgroup$ – Asaf Karagila Apr 12 '13 at 15:40
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    $\begingroup$ @AsafKaragila: Why not create a decidability tag, and have redirect it to computational-complexity or whatever? $\endgroup$ – ShreevatsaR Apr 13 '13 at 13:21
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Yes, that is what it means to reduce one problem to another: you assume you have an algorithm for the reachable code problem, and show that this would allow you to build an algorithm for the halting problem.

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