On 7/11/2021 3:23 AM, Ben Bacarisse wrote:
> Jeff Barnett <
j...@notatt.com> writes:
>
>> On 7/10/2021 7:56 PM, Ben Bacarisse wrote:
>>> olcott <No...@NoWhere.com> writes:
>>>
>>>> All of the textbooks have this same proof.
>>> You have not even read the proper proof given in the textbook that you
>>> appear to have: "An Introduction to Formal Languages and Automata" by
>>> Peter Linz.
>>>
>>>> As long as the Turing proof is based on the same idea I am set.
>>> Turing never published a proof of the halting theorem, though he
>>> certainly knew it. His 1936 paper was not concerned with halting, but
>>> with defining "commutable numbers".
>>> I think it was Martin Davis who coined the term "halting problem", but
>>> his use of the terms is not what you would recognise, and the proof is
>>> entirely different.
>>
>> It was in the mid 1960s when Davis agreed to give a few of us 20
>> somethings a quick ad hoc class on this stuff. We got together an hour
>> a day for a few weeks. To my 50+ year old recollection, the
>> no-halting-machine proof he showed us was virtually identical to the
>> one in the Linz book.
>
> Where was this? Was anything published? Did you get the feeling he was
> claiming the construction was his own invention?
There was a nationally chartered nonprofit, the System Development
Corporation, that was created from a chunk the RAND Corporation (RAND as
in Santa Monica California, not Sperry Rand) in the late 1950s. SDC
consisted of divisions of two sorts: those that experimented with
development of large defense systems such as the early warning stuff on
the DEW line in Canada to sense polar route attacks; other divisions
were two for research and technology (all computer science). By the time
I went there (early middle 1960s), the R&T divisions were major
recipients of one of the DARPA block grants.
SDC R&T did many things of interest to you, I think. For example:
Seymour Ginsburg, Gene Rose, and Shelia Greibach were doing work in
formal language theory. Seymour had a knack for identifying folks in the
field who were about to pop something significant and finding funds so
they could spend their sabbaticals at SDC so a ton of papers were
published from there in formal language theory. In a more pragmatic
vein, Erwin Book and Val Schorre developed early, industrial strength
meta compilers one of which was used in early JOVIAL development. JOVIAL
= Jules' Own Version of the International Algorithmic Language. Jules
was the head of the T in R&T and his daughter, they lived next door, was
my eldest`s first babysitter.
Other developments in R&T were an industrial weight time sharing system
that was working within a month of MIT's; early network experiments that
lead to BBN and SDC being the only two invited to submit proposals for
the original DARPA Net; I worked on something Called LISP 2 - supposed
to mature LISP and combine it with some ALGOL ideas (Marvin Minsky and
John McCarthy were official god fathers - Alan Perlis was step uncle);
and there was what I called our Philosophy initiative that sponsored
interesting visitors who stayed around for a while - Martin Davis and
Yehoshua Bar-Hillel were examples here - I think this sprung from an
interest in formal logic and automating theorem proving; early work in
computational linguistics including a project to read and internalize
the Golden Book Encyclopedia - they were still on the "Aardvark" entry
and a fairly large speech understanding effort in the 1970s.
My group consisted mostly college dropouts that were more concerned with
computers, mathematics. and science then school. About half were MIT
affiliated. We would visit Minsky and McCarthy every few months. We
would usually take a red eye to Boston, go to the top of Tech Square
were the "MAC Hackers" would meet us. The next day we would "play" with
Minsky, see Seymour Papert and others for diner and generally catch up
on what was going on in the world. That evening or the next we would
find a land rover left for us by one of my colleague`s parents and drive
through the night to New York were one of us was an NYU professor (not a
dropout). On those drives we took turns presenting seminars on logic and
general topics in CS. That was a good chunk of my "formal" education.
In any event when we found Davis among us we descended on him en masse
and demanded a class on everything he knew. He consented, partially, and
we got him for a few weeks and took a tour of a fascinating world. He
was quite into diagonal arguments; I think he thought they were pretty,
easy to teach, and easy to understand (I don't think he every met PO).
He followed good teaching practice and did not point out what was to his
credit though he did mention many others by name. I think he decided
that the group was motivated to learn and absorb the information and
that the best he could do for us was introduce us to reusable techniques
such as diagonalization and mini model theory (my terminology), and some
of the foundational results.
> My feeling is that the idea embodied in the "usual" construction was
> just floating around as a rather obvious way to persuade those readers
> who don't like the more formal proofs that textbooks tended to use.
> This may be completely wrong, but since you were around at the time,
> maybe you can shed some light on it.
I think he used whatever proof amused him at the moment, at least for
live people interactions. I think that as soon as Cantor's diagonal
proof was circulated, there was an instant recognition in the community
that this was a technique that could turn some god awful hard stuff into
something a lot more people could grasp. Of course I wasn't around in
the Cantor days but I think he thoroughly changed first guesses at how
to prove things. I think in some ways the introduction of diagonal
proofs was almost as important as defining equal size as 1-1
correspondence. One more thing, I don't think Davis had any love of
"textbook style proofs", rather I think he enjoyed pretty mathematics.
Sorry I rattled on for too long. However, that's how it came out and I
don't have a motivation or time to shorten it. I haven't eaten breakfast
yet and I'm hungry.
--
Jeff Barnett