Betti Numbers: Understanding the Topology of Spaces

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Discussion Overview

The discussion revolves around Betti numbers, which are used in topology to describe the characteristics of spaces. Participants explore the definitions and implications of the zeroth Betti number, particularly in relation to connected components, and engage in clarifying misunderstandings about the nature of connectedness in topological spaces.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that the zeroth Betti number represents the number of connected components in a space.
  • Others challenge the idea that the zeroth Betti number is always at least one, suggesting that a topological space can have multiple components or even none, as in the case of the empty set.
  • One participant humorously notes that two distinct spheres can be considered one space, implying a playful interpretation of connectedness.
  • There is a request for examples where the zeroth Betti number could be zero, indicating a need for clarification on this concept.
  • Some participants discuss the nature of the empty set, debating whether it is connected and how connected components are defined.
  • Clarifications are made regarding the terminology of connected components, with emphasis on the requirement that they must be nonempty.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation of the zeroth Betti number and its relationship to connected components. There is no consensus on whether the zeroth Betti number can be zero, as some argue it can while others maintain it cannot.

Contextual Notes

There are unresolved questions about the definitions of connectedness and connected components, particularly in the context of the empty set and how these concepts apply to various topological spaces.

realanony87
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This is probably a silly question but I am not from a maths background. I am a bit confused about betti numbers. From what i know :
The zeroth betti number is the number of connected components in space
The first betti number is the number of loops in space
the second is the number of cavities in the space

However does this mean that the zeroth betti number is always atleast one since there is always one connected component ?
 
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Why would there be always at least one connected component ? A topological space can have more than one components. One can always call the collection of two distinct spheres as one space.
 
yenchin said:
One can always call the collection of two distinct spheres as one space.
And two is at least one. :-p
 
Can you give me a case where the zeroth Betti number is 0 and explain why please?
 
Hurkyl said:
And two is at least one. :-p

Oops. I misunderstood the statement. :smile: My bad.
 
The empty space has no connected components. :)
 
adriank said:
The empty space has no connected components. :)

the empty set is connected
 
realanony87 said:
This is probably a silly question but I am not from a maths background. I am a bit confused about betti numbers. From what i know :
The zeroth betti number is the number of connected components in space
The first betti number is the number of loops in space
the second is the number of cavities in the space

However does this mean that the zeroth betti number is always atleast one since there is always one connected component ?

There is always one connected component.

But your interpretation of the first Betti numer as the number of loops is hasty. What is the first Betti number of the projective plane? What about the Klein Bottle? How many loops do they have?
 
wofsy said:
the empty set is connected

Yes, it is connected, but it has no connected components. A connected component must be nonempty.
 
  • #10
adriank said:
Yes, it is connected, but it has no connected components. A connected component must be nonempty.

true, i just thought i'd clarify the terminology
 
  • #11
adriank said:
Yes, it is connected, but it has no connected components. A connected component must be nonempty.

Totally agree :)
 

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