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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.theory
Subject: Re: Definition of real number =?utf-8?Q?=E2=84=9D?=
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Date: Mon, 01 Apr 2024 16:11:15 -0700
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olcott <polcott333@gmail.com> writes:
[...]
> Since PI is represented by a single geometric point on the number line
> then 0.999... would be correctly represented by the geometric point
> immediately to the left of 1.0 on the number line or the RHS of this
> interval [0,0, 1.0). If there is no Real number at that point then
> there is no Real number that exactly represents 0.999...
[...]

In the following I'm talking about real numbers, and only real
numbers -- not hyperreals, or surreals, or any other extension to the
real numbers.

You assert that there is a geometric point immediately to the left of
1.0 on the number line.  (I disagree, but let's go with it for now.)

Am I correct in assuming that this means that that point corresponds to
a real number that is distinct from, and less than, 1.0?

More generally, does each real number correspond to a point on the
number line, and does each point on the number line correspond to a real
number?  (The real numbers can be formally defined without reference to
geometry, but let's go with your geometric model for now.)

If so, let's call that real number (immediately to the left of 1.0) x.

Consider ((x + 1.0)/2.0).  Let's call that number y.  (The intent is to
construct a real number that is exactly halfway between x and 1.0.)

Is y a real number?  (If not, the real numbers are, unexpectedly, not
closed under common arithmetic operations.)

Is y less than, equal to, or greater than x?

Is y less than, equal to, or greater than 1.0?

Again, I am talking *only* about real numbers.

Given your past history, I do not expect straight answers to these
questions, but I'm prepared to be pleasantly surprised.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Medtronic
void Void(void) { Void(); } /* The recursive call of the void */