What is the Difference Between a Lie Subalgebra and a Subspace?

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Discussion Overview

The discussion centers on the differences between Lie subalgebras and subspaces, exploring the definitions and constraints associated with each structure. Participants seek clarification on the concepts and request concrete examples to illustrate the distinctions.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the clarity of the statement that a Lie subalgebra is a more constrained structure than a subspace, indicating the need for examples.
  • Another participant asserts that an algebra is not necessarily a vector space, highlighting a fundamental difference between subalgebras and Lie subalgebras, with the latter requiring additional structure.
  • A third participant defines a Lie subalgebra as a linear subspace that also satisfies the axioms of a Lie algebra, emphasizing the requirement of closure under the Lie bracket.
  • One participant challenges a previous claim about the nature of algebras and spaces, suggesting a misunderstanding.
  • A participant expresses confusion over the terminology, mistakenly referring to Lie groups instead of Lie subalgebras, and later retracts their comment.
  • A final participant expresses gratitude, indicating they have gained understanding from the discussion.

Areas of Agreement / Disagreement

The discussion contains multiple viewpoints and some disagreement regarding the definitions and implications of Lie subalgebras versus subspaces. No consensus is reached on the clarity of the initial statement or the definitions provided.

Contextual Notes

Some participants express uncertainty about the definitions and relationships between algebras and vector spaces, indicating potential gaps in understanding that are not fully resolved.

KarateMan
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I have a question about Lie subalgebra.

They say "a Lie subalgebra is a much more CONSTRAINED structure than a subspace".
Well, it seems subtle, and I find this very tricky to follow.

Can anyone explain this with concrete examples?

If my question is not clear, please tell me so, I will try to rephrase it.
Thanks.
 
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an algebra is nececearily not a space (understood vectorspace), so there is a big different. If you are talking about an subalgebra and a lie subalgebra. I guess you know the usual deffinition of a subalgebra, a lie subalgebra is a much more strict because a lie subalgebra needs to be a algebra + a submanifold, which is very strict.
 
KarateMan: A Lie subalgebra is a linear subspace which is a Lie algebra.
Hence, besides being a subspace, it has to satisfy the Lie algebra axioms (e.g. it has to be closed under the Lie bracket!).

mrandersdk: There are no topological requirements for Lie (sub)algebras.
 
mrandersdk said:
an algebra is nececearily not a space

This is terribly, terribly wrong.
 
so sorry always do this, i read it as lie group, why do i always do this. Sorry again.

Neglect my comment.
 
Thanks everyone. took me a while but I think I swallowed it!
 

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