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From: dbush <dbush.mobile@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: DD simulated by HHH cannot possibly halt (Halting Problem)
Date: Sat, 5 Apr 2025 17:12:19 -0400
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On 4/5/2025 1:38 PM, olcott wrote:
> On 4/5/2025 11:25 AM, dbush wrote:
>> On 4/5/2025 11:59 AM, olcott wrote:
>>> On 4/5/2025 2:42 AM, Richard Heathfield wrote:
>>>> On 05/04/2025 07:14, olcott wrote:
>>>>> On 4/4/2025 10:49 PM, Richard Heathfield wrote:
>>>>>> On 05/04/2025 00:41, olcott wrote:
>>>>>>> *Simulating termination analyzer Principle*
>>>>>>> It is always correct for any simulating termination
>>>>>>> analyzer to stop simulating and reject any input that
>>>>>>> would otherwise prevent its own termination. The
>>>>>>> only rebuttal to this is rejecting the notion that
>>>>>>> deciders must always halt.
>>>>>>
>>>>>
>>>>> typedef void (*ptr)();
>>>>> int HHH(ptr P);
>>>>>
>>>>> int DD()
>>>>> {
>>>>>    int Halt_Status = HHH(DD);
>>>>>    if (Halt_Status)
>>>>>      HERE: goto HERE;
>>>>>    return Halt_Status;
>>>>> }
>>>>>
>>>>> int main()
>>>>> {
>>>>>    HHH(DD);
>>>>> }
>>>>>
>>>>>> In other words, you operate on the principle that deciders don't 
>>>>>> have to (and indeed can't) always make a correct decision on 
>>>>>> whether an input program halts.
>>>>>>
>>>>>
>>>>> The termination analyzer HHH would be correct
>>>>> to determine that it must stop simulating DD to
>>>>> prevent its own non-termination
>>>>
>>>> Fine, but then it fails to do its job. What you are learning (albeit 
>>>> slowly) is that the termination analyser HHH can't analyse whether 
>>>> DD terminates. It is therefore not a general purpose termination 
>>>> analyser.
>>>>
>>>
>>> Introduction to the Theory of Computation 3rd Edition
>>> by Michael Sipser (Author) (best selling textbook)
>>>
>>> <MIT Professor Sipser agreed to ONLY these verbatim words 10/13/2022>
>>>      If simulating halt decider H correctly simulates its input D
>>>      until H correctly determines that its simulated D would never
>>>      stop running unless aborted then
>>>
>>>      H can abort its simulation of D and correctly report that D
>>>      specifies a non-halting sequence of configurations.
>>> </MIT Professor Sipser agreed to ONLY these verbatim words 10/13/2022>
>>
>> But not what you think he agreed to:
>>
> 
> Paraphrased as this:
> 
> *Simulating termination analyzer Principle*
> It is always correct for any simulating termination
> analyzer to stop simulating and reject any input that
> would otherwise prevent its own termination. The
> only rebuttal to this is rejecting the notion that
> deciders must always halt.
> 

Which Sipser doesn't agree with:

On Monday, March 6, 2023 at 2:41:27 PM UTC-5, Ben Bacarisse wrote:
 > I exchanged emails with him about this. He does not agree with anything
 > substantive that PO has written. I won't quote him, as I don't have
 > permission, but he was, let's say... forthright, in his reply to me.
 >