Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Topic: UNCOUNTABILITY
Replies: 59   Last Post: Dec 24, 2012 2:06 PM

 Messages: [ Previous | Next ]
 mueckenh@rz.fh-augsburg.de Posts: 18,076 Registered: 1/29/05
Re: UNCOUNTABILITY
Posted: Dec 24, 2012 3:44 AM

On 24 Dez., 01:50, William Hughes <wpihug...@gmail.com> wrote:
> On Dec 23, 1:12 am, Virgil <vir...@ligriv.com> wrote:
>
>
>
>
>

> > In article
> >  William Hughes <wpihug...@gmail.com> wrote:

>
> > > On Dec 22, 7:04 pm, Virgil <vir...@ligriv.com> wrote:
> > > > In article
> > > > <2263ace1-dfa7-466d-8341-c50692402...@v7g2000yqv.googlegroups.com>,
> > > >  William Hughes <wpihug...@gmail.com> wrote:

>
> > > > > Note, that
> > > > > subcountable
> > > > > does not mean countable.

>
> > > > I am not at all sure of what you mean by subcountable.
>
> > > A set X is subcountable if we can associate a different natural number
> > > with every element x of X, call it f(x)  In classical mathematics
> > > subcountable
> > > implies countable because f(X) must be a subset of the natural
> > > numbers.
> > > However, if we take a contructivist viewpoint, then we do not know
> > > that f(X) is a subset  (it may not be contructable).  So in
> > > constructive
> > > mathematics the fact that X is subcountable, does not mean we can
> > > find a bijection between X and some subset of the naturals, so X might
> > > not be countable.  E.g. in constructive mathematics the (constructive)
> > > reals
> > > are subcountable but not countable.

>
> > > So the fact that a set is uncountable need not mean it is "bigger"
> > > than
> > > the natural numbers.

>
> > But the constraints of your "constructive mathematics' are not required
> > in classical mathematics when not doing your constructive mathematics,
> > so are not relevant in classical mathematics.

>
> Indeed.  However the original post in this thread was concerned
> with the affect of definability on Cantor's argument.
> I note that Cantor's theorem is perfectly valid with the
> assumption that no unconstructable object exists,
> there is no (contructable) list of all (contructable) reals,
> so the reals remain uncountable.

There is no list asked for. What should that strawman be good for?
There is only the question whether the cardinality of the reals can be
larger than that of the natural numbers. The answer is no.

And in constructivism, there is nothing uncountable because everything
that can be constructed belongs to a countable set.

> My remarks are aimed
> at the obvious question, "If every constructable number is given by
> a string, is there not an injection from the constructable numbers
> to the naturals, and hence are the constructable numbers
> not countable?"  The problem is the collection of all naturals
> which represent constructable numbers is not a constructable
> subset of the naturals.

And nobody asks this silly question - except some matheologians who
want to cheat the constructivists.

>
> I am not a constructivist,

Then you should see that a subset of a countable set is a countable
set.

Regards, WM

Date Subject Author
12/19/12 george
12/20/12 Zaljohar@gmail.com
12/20/12 Barb Knox
12/20/12 ross.finlayson@gmail.com
12/20/12 Zaljohar@gmail.com
12/20/12 ross.finlayson@gmail.com
12/20/12 Graham Cooper
12/21/12 mueckenh@rz.fh-augsburg.de
12/21/12 Virgil
12/20/12 Graham Cooper
12/20/12 mueckenh@rz.fh-augsburg.de
12/20/12 Zaljohar@gmail.com
12/21/12 mueckenh@rz.fh-augsburg.de
12/21/12 Zaljohar@gmail.com
12/21/12 mueckenh@rz.fh-augsburg.de
12/21/12 Virgil
12/21/12 William Hughes
12/22/12 mueckenh@rz.fh-augsburg.de
12/22/12 William Hughes
12/22/12 Virgil
12/22/12 William Hughes
12/22/12 Graham Cooper
12/23/12 Virgil
12/23/12 William Hughes
12/24/12 Graham Cooper
12/24/12 mueckenh@rz.fh-augsburg.de
12/24/12 Virgil
12/23/12 mueckenh@rz.fh-augsburg.de
12/23/12 Virgil
12/23/12 mueckenh@rz.fh-augsburg.de
12/23/12 William Hughes
12/24/12 mueckenh@rz.fh-augsburg.de
12/24/12 Virgil
12/23/12 Virgil
12/22/12 Virgil
12/21/12 Zaljohar@gmail.com
12/22/12 mueckenh@rz.fh-augsburg.de
12/22/12 William Hughes
12/22/12 Virgil
12/21/12 Zaljohar@gmail.com
12/21/12 mueckenh@rz.fh-augsburg.de
12/21/12 Virgil
12/22/12 Zaljohar@gmail.com
12/22/12 Virgil
12/22/12 Zaljohar@gmail.com
12/22/12 mueckenh@rz.fh-augsburg.de
12/22/12 Virgil
12/22/12 mueckenh@rz.fh-augsburg.de
12/22/12 Virgil
12/23/12 mueckenh@rz.fh-augsburg.de
12/23/12 Virgil
12/21/12 Virgil
12/20/12 Virgil
12/20/12 Graham Cooper
12/20/12 Graham Cooper
12/21/12 mueckenh@rz.fh-augsburg.de
12/21/12 Virgil
12/21/12 Graham Cooper
12/21/12 Graham Cooper