Boundedness in extended complex plane

Click For Summary

Discussion Overview

The discussion revolves around the concept of boundedness in the extended complex plane (C'). Participants explore the implications of different metrics and the relationship between compactness and boundedness, as well as the effects of adding points to the space.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants question whether all subsets of the extended complex plane can be considered bounded because C' itself is bounded.
  • One participant argues that the extended complex plane is not bounded, suggesting that boundedness should mean being contained within some open ball of finite radius.
  • Another participant points out that using a spherical metric implies that C' can be viewed as a sphere without the north pole, which raises questions about how adding a point affects boundedness.
  • There is a contention regarding the relationship between compactness and boundedness, with some asserting that compactness implies boundedness, while others challenge this notion.
  • Participants discuss the necessity of defining a metric to assess boundedness, with one emphasizing that without a metric, the concept of boundedness cannot be applied.
  • One participant expresses confusion about the implications of adding a point to the space and how it affects the boundedness of sets.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the extended complex plane is bounded. Multiple competing views remain regarding the definitions and implications of boundedness, compactness, and the role of metrics.

Contextual Notes

There is an ongoing discussion about the definitions of boundedness and compactness, the role of metrics in these definitions, and the implications of homeomorphism on the concept of size. Some assumptions about the nature of the extended complex plane and its subsets remain unresolved.

matness
Messages
90
Reaction score
0
what does it mean to be bounded in extended complex plane (say C')?
can we say all subsets of C' is bdd because C' itself bdd?
 
Physics news on Phys.org
The extended complex plane, in my opinion, is not bounded. Bounded ought to mean, for a metric space, contained in some open ball of *finite* radius.
 
to be more explicitly
1)if i use spherical metric( i.e. i think on the sphere ) then C equivalently sphere without north pole is bdd .
why adding one point namely inf. changes boundedness
2)another point of view : i can prove C' is compact so it is closed and bdd

maybe iam missing somethings but i can't find them
 
You are missing something. Just because the extended complex plane is homeomorphic to a sphere does not mean it is bounded. The real line is homeomorphic to (-1,1), and I doubt you think that R is bounded.

Compact is not the same as closed and bounded. Cu{oo} is not a subset of R^3. It is homeomorphic to a subset of R^3, but 'size' is not invariant under homeomorphism.
 
i didnt say it is bdd because S^2 is in R^3. for example an open ball with center at inf. and radius some small quantiity (say eps.) contains
{ z€C s.t. |z|>eps } U {inf} by the way it is defined .
here what i stuck : why adding only one point(o.k. now it is a ball)
changes boundedness.
if you say i can't take inf. as a center i won't insist on that
but i am pretty sure i saw somewhere compactness implies bddness.
 
It is not bounded. Boundedness applies to subsets of metric spaces. What is the metric you're using? How far from 2 is infinity? Compact is not the same as closed and bounded. The point is you cannot have balls of infinite radii. I said nothing about infinity being or not being the centre of an open set, but as it happens, you cannot have infinity as the centre of a ball defined by the metric because there is no metric given.
 
Last edited:
now there is a new thing that comes to my mind.
the reason that i cannot say such a thing because a set is bdd whenever that this set is a subset (proper)?.otherwise we would say C is bdd in C
OR R is bdd in R e.t.c.
still wrong?
 
You can't say it is bounded because you need to use a metric to define the size of your sets. You haven't put a metrc on the extended complex plane.
 
:) i got it with one second difference . so i stop bothering.
thanks
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 3 ·
Replies
3
Views
4K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 1 ·
Replies
1
Views
4K
  • · Replies 6 ·
Replies
6
Views
6K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 4 ·
Replies
4
Views
4K
  • · Replies 3 ·
Replies
3
Views
2K
Replies
3
Views
2K