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13.3.  The Halting Problem

Description: Let M be a Turing Machine and S be a sequence of 0's and 1's.

Question: Is there a Turing Machine that can decide if M ever halts when given S as input?

Notice that any machine solving this problem receives as input two strings of 0's and 1's, the first one encoding a TM M and the second a string that will play the role of input to M. The output of this hypothetical TM should be ``yes'' or ``no'', depending on whether the machine M stops or not when given the second input string as input.

One could think that some kind of universal TM could solve this problem; nevertheless, as we show below, this problem is noncomputable.

Let us suppose that there is a TM H that can solve

The Halting Problem.

If this is the case, let us build another TM H' that computes as follows:

--       machine H' takes an input x, duplicates it
         and gives the resulting strings to machine H.
--       if the machine H answers ``yes'', then machine H'
         enters an infinite loop -- it runs forever -- if H
         answers ``no'', then machine H' stops.

Machine H' is shown in the following figure:

Turing machine H'

It should be clear that machine H' can be built if machine H can. Now let us argue why it is that H' is not a feasible machine; that is, H' is theoretically possible but in reality is impossible to implement. Since we know that a TM can be encoded into 0's and 1's, let us give H' its own encoding as input. Machine H' duplicates its own code and gives it to machine H; if machine H answers ``yes'' -- to mean that machine H' halts when given its own encoding as input -- machine H' loops forever. If machine H answers ``no'' -- to mean machine H' when given its own encoding loops forever -- machine H' stops.

We can then infer that machine H' stops when given its own encoding if and only if it does not stop when given its own encoding. Clearly, this is a contradiction. Hence machine H cannot exist.

Such as ``The Halting Problem'', there are many other noncomputable problems.

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Created by unroff & hp-tools. © by Hans-Peter Bischof. All Rights Reserved (1998).

Last modified: 27/July/98 (12:14)