In article
<
5d2cee38-df15-4999...@g1g2000pri.googlegroups.com>,
Graham Cooper <
graham...@gmail.com> wrote:
> On Nov 28, 11:47 am, Barb Knox <s...@sig.below> wrote:
> > In article
> > <
1cfc7aec-72ee-4f29-ae67-78c9e3261...@u5g2000vbd.googlegroups.com>,
> >
> > "christian.bau" <
christian....@cbau.wanadoo.co.uk> wrote:
> > > On Nov 26, 10:07 pm, Graham Cooper <
grahamcoop...@gmail.com> wrote:
> >
> > > > It's not trivially incomplete because you cannot calculate an anti-
> > > > diagonal without realising your construction process is flawed.
> >
> > > You'll need to explain that. We are talking about an infinite matrix,
> > > and there is no such thing as the antidiagonal of an infinite matrix.
> >
> > You apparently allow infinite matrices in your mathematics. Therefore
> > you must surely also allow infinite sequences, since every row of an
> > infinite matrix is an infinite sequence. Since you allow infinite
> > sequences which are rows, you must surely also allow infinite sequences
> > which are diagonals. If you allow infinite diagonal sequences, how can
> > you possibly disallow infinite anti-diagonal sequences?
> If you LIST (enumerate) a countable SET
> then you establish a diagonal.
>
> For the purposes of what a countable set contains,
> the enumeration is insignificant.
I am relaxing my usual practice of not (knowingly) replying to you,
because here you actually make a sensible mathematical point.
We agree that sets are unordered, so a given countable set S does not
come with any intrinsic enumeration.
BUT (and here comes some maths), consider what it *means* for a set S to
be countable. It means that there is at least one bijection between the
set of natural numbers and S. So, let f be any one of these bijections
(it does not matter which one). The bijection f imposes an enumeration
on S, namely f(0),f(1),f(2),f(3),...
So, given any countable set, there is at least one enumeration of it (in
fact there are vastly many). Let S be any countable set of digit
strings representing real fractions between 0 and 1. Use any one of the
enumerations of S to construct an anti-diagonal digit string. That
string will not be in S.
Different enumerations will produce different anti-diagonals, but the
key fact is that *every* enumeration produces an anti-diagonal. So any
enumeration suffices to generate an anti-diagonal digit string which is
not in S.
[snip]