WM <
askas...@gmail.com> writes:
(AKA Dr. Wolfgang Mückenheim or Mueckenheim who teaches "Geschichte des
Unendlichen" at Hochschule Augsburg.)
This reply is intended only for students who might fall for this sort of
thing. Some do, apparently... WM is impervious to logic (after all, he
explicitly assumes his own conclusion here) so he's never going to
learn.
> Assuming that countability is a sensible notion, I show that all real
> numbers belong to a countable set of cardinality |ω^ω| = ℵo.
>
> The virtue of this method is that it avoids to squander the natural
> numbers: The entries of the Cantor-list are not enumerated by all
> natural numbers but by powers of 2. The antidiagonals of this list
> (never more than a countable set)
Here WM explicitly assumes a fact logically equivalent to the conclusion
he is supposedly proving. There are, in fact, uncountably many
anti-diagonals. If there were, as WM incorrectly assumes, only
countably many anti-diagonals there would be no need for the powers of
primes construction: number any initial list of with the even numbers
and then enumerate the supposedly countable anti-diagonals using the odd
numbers.
So how can we show that there are /always/ uncountably many
anti-diagonals for any enumeration of the reals? If you a student,
particularly one of WM's students, pause here and see if you can prove
this fact.
.
.
.
.
.
OK, is this how you did it...
The conventional way to construct the anti-diagonal is to change digits
by, say, incriminating each digit mod 10, with the technical detail of
avoiding using a 9, so we might map both 8 and 9 to 0.
Here's an example of list of reals in [0,1] indexed by WM's powers of
two:
2: 0.612087234...
4: 0.971023145...
8: 0.651825344...
16: 0.443511672...
...
The diagonal is 0.6715... so the "incriminating" anti-diagonal starts
0.7826... But we could also have a "decrementing" anti-diagonal that
starts 0.5604...
But we could also choose to either increment (-) or decrement (-) at
each digit and still have an anti-diagonal. I.e. there are at least as
many anti-diagonals as there are infinite sequences of +s and -s. Can
you finish off the proof now?
Of course you can! Map + to 1 and - to 0 and we have the binary digits
of the reals in [0,1], i.e. WM is assuming the result he is trying to
prove.
> are enumerated by the powers of
> 3. If a new list is created including these antidiagonals then its
> entries are enumerated by the powers of 5 and its antidiagonals (never
> more than a countable set) are enumerated by the powers of 7. This is
> going on, in every step using a new prime number. According to Euclid
> the set of prime numbers will never get exhausted.
So, students, if WM presents this nonsense to you, be brave and counter
it in all three ways:
1. The plain counter-argument: the result of this construction is,
supposedly, is a function wm from N to [0,1] yet the anti-diagonal
constructed from flipping digit n of wm(n) is provably not in the image
of wm.[1]
2. The complexity argument: if his assumption about there being only
countably many anti-diagonals was correct, why did he use such a complex
argument?
3. The why the argument is wrong argument: it assumes the conclusion.
There are uncountably many anti-diagonals and every new prime enumerates
only countably many of them. The resulting list has, yet again,
uncountably many anti-diagonals.
Be prepared for the resulting snow-storm of waffle.
--
Ben.