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Topic: Cantor's first proof,
Replies: 3   Last Post: Nov 16, 2012 4:36 AM

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Ben Bacarisse

Posts: 1,972
Registered: 7/4/07
Re: Cantor's first proof,
Posted: Nov 15, 2012 5:13 PM
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Uirgil <uirgil@uirgil.ur> writes:

> In article
> <>,
> WM <> wrote:

>> On 15 Nov., 16:44, Ben Bacarisse <> wrote:
>> > WM <> writes:
>> > > On 15 Nov., 12:41, Ben Bacarisse <> wrote:
>> >
>> > >> Ah, simple.  N is an "impossible set" (not a term he defines but why get
>> > >> bogged down in detail like that).  The argument is simple: if you try to
>> > >> enumerate N you can construct a diagonal that is not in N; indeed it's
>> > >> not even a natural number.

>> >
>> > > A number is identified by its digits. Which digit shows you that the
>> > > diagonal is not a natural number?

>> >
>> > Not all sequences of digits correspond to natural numbers, in particular
>> > infinite ones such as the diagonal your construction creates.

>> What digits do you need to know that that the number is unnatural? Can
>> you know these digits?
>> Regards, WM

> One can know that any non-zero digits following a decimal point (or
> other radix point) in a number make it not a natural number.

There's a limit to how much time should be sent on stuff like this, but
that part I was referring to was where WM applies a diagonalisation
argument to a list of natural numbers, modifying digits from right
adding zeros as needed. The result is therefore not a natural number.


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