Topology - Boundary of a ball without a point

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Discussion Overview

The discussion revolves around the concept of the boundary of a ball in the context of complex analysis, particularly focusing on the implications of removing a point from the ball. Participants explore the relationship between the analytic function and the boundary of the ball, seeking to clarify the nature of the boundary when the center point is excluded.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the boundary of the ball, defined as ##Ball_1(0)##, is ##{0}## or ##{\emptyset}## when considering the function ##f(z)=\frac{1}{z}##.
  • Another participant asserts that in real n-dimensional space, the boundary consists of all points at a distance of 1 from the origin, expressing confusion over the relevance of the function to the boundary definition.
  • A later reply indicates that removing the point zero from the ball leads to a boundary that includes both ##|z|=1## and ##|z|=0##, suggesting a specific interpretation of the boundary in complex analysis.
  • One participant expresses skepticism about the reliability of the information provided by a trusted source, implying that the understanding may not be sufficient for academic evaluation.
  • Another participant agrees that the original phrasing was confusing and clarifies that removing the point zero does indeed make it a boundary point.
  • A final contribution states that in a metric space, the boundary of a set is defined as the set of points at a distance of 0 from the set.

Areas of Agreement / Disagreement

Participants express differing views on the implications of removing the point zero from the ball and its effect on the boundary. There is no consensus on the correct interpretation of the boundary in this context.

Contextual Notes

Participants reference complex analysis and metric spaces, but there are unresolved assumptions regarding the definitions and implications of boundaries in these contexts.

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TL;DR
Let us say we have f analytic in Ball_1(0).
which means, radius 1, starting at z_0 = 0 point.
If I want to find the boundary of Ball_1(0).
Will the boundary be {0} or {empty}?
Not homework, just an intuition to understand f(z)=1/z function ( for example ) better.
Let us say we have f analytic in ##Ball_1(0)##. which means, radius 1, starting at ##z_0 = 0## point. If I want to find the boundary of ##Ball_1(0)##. Will the boundary be ##{0}## or ##{\emptyset}##? Not homework, just an intuition to understand ##f(z)=\frac 1 z## function ( for example ) better.
 
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As stated, assuming real space of n dimensions, the boundary consists of all points at a distance 1 from the origin. Your statement is confusing? What has f to do with the ball boundary?
 
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Likes   Reactions: wrobel
Hi, it does not matter anymore.
Asked someone from my course which I trust and he said to me the answer :)
Basically, As I said.
At complex analysis.
If you have the Ball I said, with radius 1 and it beginning at point zero.
If you create the set of that ball without the point zero, then the boundary will be the unision of ##|z|=1## and ##|z|=0##
Thanks though
 
I suspect that "someone which you trust" will get the same unsatisfactory mark at an exam as you :)
 
wrobel said:
I suspect that "someone which you trust" will get the same unsatisfactory mark at an exam as you :)
They're not wrong though, op just phrased it confusingly. Op is asking if you remove the point 0 from the ball, is it now a boundary point? The answer is yes it is.
 
In a metric space, the boundary of a set S is the set of points at distance 0 from the set.
 

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