A question regarding quotient space

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Discussion Overview

The discussion revolves around the properties of quotient spaces, specifically whether the quotient space V/N includes the subgroup N itself and the implications of this inclusion. Participants explore definitions and relationships between V and V/N, touching on concepts from advanced calculus and group theory.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asserts that V/N includes N itself because the definition of cosets allows for the case when α=0, leading to the conclusion that V/N is equal to V.
  • Another participant agrees that N is included in V/N but clarifies that this means N is an element of V/N, not a subset, and argues against the idea that V could equal V/N.
  • A third participant emphasizes that elements of V/N are sets of cosets, suggesting that N acts as the "zero vector" in this context.
  • A fourth participant references Lagrange's theorem to argue that an isomorphism between V and V/N can only occur if the size of N is 1, indicating a limitation on the relationship between these spaces.

Areas of Agreement / Disagreement

Participants express differing views on whether V/N includes N as a subset or merely as an element. There is also disagreement regarding the equality of V and V/N, with some asserting it cannot hold true while others suggest potential isomorphism under certain conditions. The discussion remains unresolved regarding these points.

Contextual Notes

Participants reference definitions and theorems that may depend on specific mathematical contexts or assumptions, such as the nature of the spaces involved and the properties of the subgroup N.

Who May Find This Useful

This discussion may be useful for students or practitioners in advanced calculus, algebra, or topology who are exploring the concepts of quotient spaces and their properties.

yifli
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Does quotient space V/N include N itself?

I think it does because:

V/N includes all the cosets of N, and a coset of N is defined as N+\alpha={\epsilon+\alpha: \epsilon \in N}. This definition is from Advanced Calculus by Loomis & Sternberg and it does not say \alpha cannot be 0. So V/N includes N itself when \alpha=0.

That said, I think V/N is equal to V.

Am I wrong?

Thanks
 
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yifli said:
Does quotient space V/N include N itself?

Yes, but in the sense that N\in V/N, NOT in the sense that N is a subset of V/N!

That said, I think V/N is equal to V.

Am I wrong?

I'm afraid so, yes. The elements of V/N are subsets of V. So there's no way that V could ever equal V/N! (it could be isomorphic though, but you're not talking about that here).

For example, if V=N=\mathbb{R}, then V/N=\{\mathbb{R}+\alpha~\vert~\alpha\in \mathbb{R}\}=\{\mathbb{R}\}.
So V has infinite number of elements and V/N has only 1 element...
 
You have to keep in mind what the elements/vectors of V/N are. The vectors in this space are entire sets of their own. If V/N is your quotient space, then N is in the space, acting as the "zero vector"
 
In addition, by , I think Lagrange's theorem, |V/N|=|V|/|N| , so the only way you
could have some kind of isomorphism between V and V/N is if |N|=1.

Maybe a good way of seeing things is that you have enlarged the identity
element in your group ( or other algebraic object) , from the identity element
to the whole subgroup (ideal, etc.) N.
 
Last edited:

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