news:80ab7315-3870-42f0...@q2g2000vbv.googlegroups.com...
> On 5 Jun., 19:48, "Mike Terry"
> <
news.dead.person.sto...@darjeeling.plus.com> wrote:
> > "WM" <
mueck...@rz.fh-augsburg.de> wrote in message
> >
>
> > Or put another way, every neighbourhood of x contains a point belonging
to a
> > cluster.
> >
> > I think I should explain more carefully what this means. It means that
if
> > you give me a neighbourhood N of x, THEN I can find a cluster C that
> > intersects N. Note that my choice of C is made AFTER you give me N, so
I'm
> > free to choose a different C for each different N you give me. It does
not
> > mean that there is a single cluster C such that C intersects every
neighbour
> > hood of x. The order of qualifiers is important!
>
> But the real axis is a static, timeless object. There you have no hide
> and seek play but a point either is disconnected from all points that
> are smaller and larger, or it is not. I dont give you any N and no
> quantifyer, but I ask whether the point x is single like the point 0
> in the definition:
> f(0) = 0 and f(x) = 1 for all x =/= 0.
> If this is the case, then I know that the point 0 is a singleton and
> interrupts a removed interval.
Right I've explained the exact sense in which x is a "singleton" several
times.
If you don't like that definition call it something else - a "separated"
point or whatever. Names do not change the truth of things.
If you like you could make your own definition of singleton. Let's see:
Def. x is a singleton if it's like the point 0 in the definition:
f(0) = 0 and f(x) = 1 for all x =/= 0.
Umm, no. Sorry, that's not going to do :-) Much to vague!
Hmmm, although I was being sarcastic [sorry], I've suddenly had an idea
about what you really might mean by "singleton"! How about this:
Def. x is a singleton if there is a punctured neighbourhood N' of x
such that N' is a subset of the union of all the clusters.
or, equivalently if you don't like the language of neighbourhoods etc. from
topology:
Def. x is a singleton if there is an interval measure > 0
containing x as an interior point such that all the
remaining points of the interval (other than x) belong
to clusters.
Yep, that makes some sense, and I believe normally we would describe such
points as "isolated". If x is "isolated", then it follows it is the end
point of two clusters, one above it and one below. (That seems to fit in
with what you've been saying?)
...So, let me summarise where we've got to!
We start with the interval [0,1] and "remove" intervals of length 10^-n
centred around the corresponding rationals {q_n}, and consider the
complement that remains.
a) The connected components of the complement consist of singleton
sets {x}, for certain irrational values of x. These points are
"separated" from each other, in the sense I've described
previously, but are not all "singletons interrupting an interval".
b) None the less, we will say the set with just one member {x} is a
"singleton set".
[Sorry, that's standard terminology in set theory that everybody
uses, and I see no value in changing that, even for you!]
And we might as well then admit to call the singleton set {x}
a singleton interval [x,x]. (But if you like we won't call
x a singleton point in any other context...)
c) SOME of the x are "isolated", and can be described as
"singletons interrupting an interval". These x have clusters
immediately above and below them. [I'd rather just call these
"isolated", and reserve the use of "singleton" to it's set
theory meaning.]
d) Other x are not adjacent to any clusters, but instead have infinite
sequences of clusters above and below them, the clusters converging
to the point x. So these x are not "isolated" and cannot be
described as "singletons interrupting an interval". If you
like we could avoid calling such x "singletons", although they
are unarguably "singleton sets", being sets consisting of
just one member!
You seem to doubt the existence of points mentioned in (d)? If so, you
should prove there are no points of this kind!
> >
> > > Intervals around rationals.
> >
> > You could say this of course, but it does not mean the singletons are
> > "directly adjacent" to any clusters - there can be infinite sequences of
> > clusters approaching closer and closer to x. [as suggested in my C_n,
D_n
> > example below]
>
> And by this ecstasy of infinity we numbers get a bit muddled up and
> countably many intervals are cut by uncountably many singletons
> without getting uncountably many pieces. A fine result.
Yeah, that's business as usual in the world of rationals and reals! I
remember thinking this was neat when I was about 12, but I guess we get used
to such results and lose our initial sense of wonder as we can just "see
it's true" and get on with it. Like, how can there be infinitely many
naturals, and yet...and yet... every natural number is finite!!!
Sometimes posters come to sci.math and they're genuinely confused about
simple things like this, having studied too much philosophy or something I
guess. After several posts trying to explain, I feel like saying "hey if
you can't just SEE this is the case, e.g. that AxEy is not the same thing as
EyAx, or whatever, then maths isn't going to be for you - it's only going to
get harder!!" :-)
> >
> > If you don't think such a situation is possible, and that every x is the
> > endpoint of a cluster then by all means provide a proof of this - but an
> > assertion is not the same as a proof. And repeatedly stating "there
must be
> > something that makes it a singleton" is not a proof! :)
>
> Any uncovered irrational is a singleton only when it interrupts an
> interval.
Maybe this difficulty has been cured with my terminology change above? SOME
of the uncovered irrationals are "isolated" and can be said to "interrupt an
interval".
Others are not "isolated", and do not "interrupt an interval". Neverless,
they are still in the complement set. And their connected components in the
complement are "singleton sets", but I'll try not to call the point x itself
a singleton point if you like...
>
> > > > No, there needn't be a cluster lying to the right of x. Or rather,
there
> > > > needn't be any cluster directly adjacent to x.
> >
> > > Then x is not a singleton.
> >
> > Why not? This is exactly what you need to prove rather than just
assert.
>
> It is not necessary to prove it. It is the definition. A singleton is
> a set of 1 element that is not covered, so it is 1 point, and there
> must be an interval with measure > 0 at both sides that has been
> covered. Remember, the real axis is a static object. There is no hide
> and seek play adequate.
Fair enough if you want to make this definition, but it doesn't change the
underlying maths! So with your definition I could agree that a singleton
{x} from the complement has a cluster immediately above and below it.
But... now the problem is that there are connected components of the
complement that are "sets with one element" {x}, but are lacking any
"interval with measure > 0 at both sides that has been covered".
So you still end up having to prove the same thing, but using different
definitions. :)
To summarise: you need to provide a proof that all x in the complement have
an interval with measure > 0 at both sides that has been covered.
>
>
> > No, this is your same mistake again: assuming that each singleton must
be
> > the endpoint of a cluster. [I see that several others have said the
same
> > thing to you in other threads, so this seems to be the key sticking
point.]
>
> If it is not the endpoint, then there is a point between it and the
> endpoint, no?
Here is my description of the situation from an earlier post:
We have infinite sequences of clusters
C_1, C_2, C3,... and D_1, D_2, D3
We have the "uncovered point x"
We have C_1 < C_2 < C_3 < ... < x < ... < D_3 < D_2 < D_1
x is the least upper bound of Union(C_n) and
x is the greatest lower bound of Union(D_n)
So x is firmly "sandwiched" between the sequences of clusters above and
below it.
Now let's try to answer your question: "If x is not the endpoint, then
there is a point between it and the endpoint, no?"
The obvious problem with your question is "between it and WHAT endpoint?"
There is no "the" endpoint so your question doesn't make sense...
Well, in my diagram, x is not the endpoint of D_3 - let's focus on that.
For sure there is a point between D_3 and x. In fact, between D_3 and x
there are both entire clusters [D_4, D_5, etc.] and there are further
"uncovered" points, viz the endpoints of the clusters D_4, D_5, etc..
So do you want me to answer "yes" to your question for this reason?
Otherwise you need to think more carefully what you're asking. Specifically
when you say "between x and the interval", I would like to know WHAT
interval you're referring to.
> >
> > > And every interval is extended, i.e., it is not only a point.
> >
> > Well, most people count the closed interval [x,x] as an interval
consisting
> > of a single point.
>
> That is correct. But all intervals that cover the rationals q_n have
> the measure 10^-n. None of them is a point.
Sure - all the "covering" or as I sometimes call them "removed" intervals
have length > 0.
However every "remaining" interval in the complement has length = 0, i.e.
the interval is a singleton set (as the term is used in set theory, viz a
set with just one element).
>
>
> > Obviously such intervals aren't extended. However, all
> > the intervals removed from the line (centred on the rationals) were by
> > definition extended. What remains could be said to be singleton
intervals
> > (which aren't extended), or don't describe the {x} as intervals if you
don't
> > want [x,x] to be counted as an interval...
>
> That's ok. They are counted as intervals. (Even the empty set is
> counted as an interval.) But they are only then points, if they are
> the only uncovered points in a neighbourhood where all other points
> are covered.
Yeah - but this is the case for ALL the remaining points in the complement
set!
At least I know it's OK for me to call them intervals, although there's some
remaining doubt about the terminology "singleton". :(