Turing Machine Halting Problem


Input − A Turing machine and an input string w.

Problem − Does the Turing machine finish computing of the string w in a finite number of steps? The answer must be either yes or no.

Proof − At first, we will assume that such a Turing machine exists to solve this problem and then we will show it is contradicting itself. We will call this Turing machine as a Halting machine that produces a ‘yes’ or ‘no’ in a finite amount of time. If the halting machine finishes in a finite amount of time, the output comes as ‘yes’, otherwise as ‘no’. The following is the block diagram of a Halting machine −

Halting Machine

Now we will design an inverted halting machine (HM)’ as −

  • If H returns YES, then loop forever.

  • If H returns NO, then halt.

The following is the block diagram of an ‘Inverted halting machine’ −

Inverted Halting Machine

Further, a machine (HM)2 which input itself is constructed as follows −

  • If (HM)2 halts on input, loop forever.
  • Else, halt.

Here, we have got a contradiction. Hence, the halting problem is undecidable.

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