"WM" <
muec...@rz.fh-augsburg.de> wrote in message
news:c7bfbccd-fdb3-4fbe...@e20g2000vbm.googlegroups.com...
> On 24 Jun., 02:12, "Mike Terry"
>
> > I think WM believes there are only countably many singletons, and his
> > argument for this was that each singleton is adjacent to a cluster, so
there
> > is some kind of correspondence between them. He's been told this is
wrong
> > of course, and has not provided any proof of this correspondence,
>
> The proof is that an uncovered irrational is a singleton, i.e. it is
> adjacent to intervals I_n. Otherwise it was not a singleton.
No - that is not the definition of a singleton. I don't think you
understand what is meant by describing the uncovered irrationals as
singletons, even though you introduced the wording - if you did you would
see the need for a proof of a claim such as this, rather than repeating the
claim over and over.
It is disappointing that when challenged to prove a claim, your proof turns
out to be:
Claim: Every uncovered x is the endpoint of some cluster.
Proof: 1. x is a singleton
2. all singletons are adjacent to intervals I_n.
3. therefore x is adjacent to an interval I_n.
4. (i.e. x is the endpoint of the interval I_n.)
Do you really believe that constitutes a mathematical proof? (In case you
genuinely do, the problem is with (2), so please come back and prove (2).)
>
> > so there's
> > not much to be said, other than he's wrong! Of course each cluster is
> > "adjacent" to two singletons, but not vice-versa,
>
> Do you believe in singletons that are adjacent to other singletons
> without a rational between them? That would simply be the consequence.
No I don't, and no it wouldn't. (Not that it matters what I think - we're
waiting for you to provide a proof of your claims, which clearly you will
never do.)
>
> > and it's been explained to
> > him how for certain irrationals there are *sequences* of clusters which
> > converge to the irrational, with no single cluster being adjacent to
it...
>
> Cantor's axiom says that every irrational x is a point on the real
> axis. Whether this point is a singleton, is not at all determined by
> distant intervals or clusters, but only by intervals that are adjacent
> to x. And if there are no such intervals, then x is not a singleton.
No that's plain wrong - x "is a singleton" if x belongs to the complement,
and there is no larger connected component containing x within the
complement. The situation of having a larger connected component within the
complement can be prevented by there being a *succession* of clusters
approaching x arbitrarily closely, but no cluster being adjacent to x.
> >
> > Anyway, when he asks "what makes a singleton a singleton?" I think he
wants
> > you to say "the clusters that have been removed", and then he will say
> > "aha - that means that there must be clusters adjacent to it!" which of
> > course is just muddled thinking...
>
> Otherwise you must believe in singletons that are not adjacent to
> intervals. Or do you see another alternative?
>
> But try to comprehend: Cauchy sequences and clustering and the like do
> not help you, if Cantor's axiom is valid, which says that every real
> number corresponds to a point of the real axis. Then *only* the direct
> surrounding decides about the character of a singleton.
There is no "direct surrounding" for a single point x on the real line, in
the sense of there being points directly adjacent to x. What we have at x
is a "neighbourhood system" which characterises closeness to x in a sense.
What is true, is that every neighbourhood of an uncovered x contains a point
belonging to one of the clusters. So we can say this is what makes x
isolated.
Note the order of quantification here: given a neighbourhood of x there is
a cluster intersecting that neighbourhood. This cluster depends on the
choice of neighbourhood. It is NOT saying that there is a single cluster
that intersects with every neighbourhood of x. (The latter is true for
some, but not all the uncovered x.) I only mention this because I know from
other posts that you are confused on this point...
> In fact there
> is a neighbourhood for every singleton, such that the neighbourhood
> does not contain any further uncovered irrationals, i.e. any further
> singleton.
No, this is wrong. It is essentially what you've been asked to prove, but
have so far only claimed. Of course, feel free to submit a proof of this
claim if you believe it to be true, or withdraw the claim if you realise you
were mistaken. Otherwise just repeating it over and over will result in
people concluding you're a crank...
Mike.