WM <
askas...@gmail.com> writes:
(AKA Dr. Wolfgang Mückenheim or Mueckenheim who teaches "Geschichte des
Unendlichen" at Hochschule Augsburg.)
> Ben Bacarisse schrieb am Freitag, 2. September 2022 um 18:42:47 UTC+2:
>> WM <
askas...@gmail.com> writes:
>>
>> > Ben Bacarisse schrieb am Freitag, 2. September 2022 um 18:00:32 UTC+2:
>> >> WM <
askas...@gmail.com> writes:
>> >>
>> >> > That means there is a bijection between visible natnumbers
>> >> > and visible fractions.
>> >> Your textbook does not explain what a visible number is
>> >
>> > My textbook, like claisscal maths deals only with visible numbers. No
>> > reason to mention this. Further in 2015 when it was published I did
>> > not know about dark numbers.
>> I have always been talking about the maths in your book. Did I not make
>> that clear enough? Sorry.
>>
>> If your book is no longer adequate for this task, please say so, but I
>> hope it is because you are not good at answering direct questions, but
>> the book is there for all to see. What I wanted to know is:
>>
>> (a) Is k (now cut) a function from NxN to N as the set N and the term
>> function are defined in your book?
>
> In my book there are, as I clearly stated in the introduction, only
> potentially infinite sets. That are the visible numbers which are
> sufficient to do classical mathematics.
>
>> (b) Is k surjective (as defined in your book)?
>>
>> (c) Is k injective (as defined in your book)?
>>
> Of course.
Great. So k /is/ a bijection (with everything -- functions, bijections,
N, NxN -- as defined as in your book).
> But Cantor talked about actual infinity. I notice that you don't wish
> to consider Cantor's mapping.
Let's see how far we can get just with WMaths. The bijection you wrote
k(n,m) = (m + n - 1)(m + n - 2)/2 + m
will let us write the effect of a sequence of swaps in terms of a
sequence of WMaths functions. We may hit the rocks and find that WMaths
can not take us where we want to go, but let's try. Are you up for
that?
> There we have the complete matrix of positive fractions and the
> complete column of integer fractions:
>
> 1/1, 1/2, 1/3, 1/4, ...
> 2/1, 2/2, 2/3, 2/4, ...
> 3/1, 3/2, 3/3, 3/4, ...
> 4/1, 4/2, 4/3, 4/4, ...
> 5/1, 5/2, 5/3, 5/4, ...
> ...
>
> Cantor's approach is modelled by exchanging X's and O's in
>
> XOO...
> XOO...
> XOO...
> ...
>
> until all O's have disappeared. Do you agree?
Here are the cells as numbered by your bijective mapping:
1, 3, 6, 10, 15, ...
2, 5, 9, 14, 20, ...
4, 8, 13, 19, 26, ...
7, 12, 18, 25, 33, ...
11, 17, 24, 32, 41, ...
...
With Os at 3, 5, 6, 8, 9 and so on. Let's use 1 and 0 for Xs and Os so
the matrix can be written as a function from N to {0,1} (as defined in
your book). Since these are functions of N, let's just write the first
few values rather that trying to find the ever-so messy formulas for
where there are 1s and 0s:
M_0(n) = 1, 1, 0, 1, 0, 0, 1, 0, 0, 0, 1, ...
Now what are the first few swaps? The logical choice would be to swap
the first 0 with the first following 1 so that
M_1(n) = 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 1, ...
M_2(n) = 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, ...
M_2(n) = 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, ...
...
I think this sequence of functions makes the pattern nice ans clear and
all the functions are just the sort of functions you manipulate in your
book. Would you like these turned back into X and O matrices? I don't
think it helps, so I haven't bothered.
Now the only meaning for the endless swapping that makes sense to me is
a limit, and fortunately you tell your students how to prove that
lim_{n->oo} M_n = M_1
where M_1 is the constant function M_1(k) = 1. You give a nice
definition of convergence for sequences of functions in your textbook,
and the function sequence M_n does indeed converge to that limit.
> Or did I misinterpret Cantor?
I don't know. Do you have a citation for his discussion of this matrix?
I does not have the neat structure of most of his arguments so I don't
think you've got it right.
> Or do you prefer not to get involved in that question because it could
> be too dangerous to become a heretic matheologian?
I'm not sure what you think is dangerous. The WMaths limit of the
WMaths function sequence M_n has no 0s in it, and that seems like the
only reasonable meaning for the effect of an endless sequence of swaps.
It seems that WMaths is up to the task. If that makes me a heretic, I
really don't care.
> I could understand your hesitation.
My only hesitancy was due to your flip-flopping about what is and is not
provable. If k (as you defined it) is not a bijection (as you define
the term in your book) between NxN (as you define it in your book) and N
(as defined in your book) then I could not even start formalising, in
WMaths, what this endless sequence of swaps might look like. Even now,
a feel sure a flip or a flop coming on... You find it hard to stick to
talking about WMaths for some reason.
But I note that /you/ still hesitate to show how a function is proved to
be bijective in WMaths. Well, hesitate is a rather weak term because I
have been asking, on and off, for /years/. It's almost as if you don't
know or are, for some reason, afraid to reveal how simple it is in
WMaths. I'm not expecting your hesitation to vanish, but I live in
hope so I am asking again.
--
Ben.