Does every Hilbert space have an identity?

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Discussion Overview

The discussion revolves around the concept of identity in Hilbert spaces, specifically whether every Hilbert space has an identity element. It also touches on the definition of closed subspaces within Hilbert spaces.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the meaning of a closed subspace in a Hilbert space, providing definitions related to vector spaces and norms.
  • Another participant suggests that a Hilbert space has an additive identity, specifically the element 0.
  • There is a clarification sought regarding the meaning of "identity," with one participant indicating they mean the multiplicative identity, which is 1.
  • Participants express confusion about the concept of multiplication in the context of Hilbert spaces, with one noting that it may not be applicable.
  • There is a light-hearted acknowledgment of the perceived simplicity of the questions posed, with a reminder that there are no stupid questions.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the existence of a multiplicative identity in Hilbert spaces, and there is uncertainty regarding the applicability of multiplication in this context.

Contextual Notes

The discussion lacks clarity on the definitions and contexts in which "identity" is being used, particularly distinguishing between additive and multiplicative identities.

LikeMath
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I am sure that my questions are stupid. If we have a Hilbert space H, what do we mean by the closed subspace of H. Also, Does every Hilbert space have an identity? :P.

Could anyone please clean to me these things .
Thanks!
 
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LikeMath said:
I am sure that my questions are stupid. If we have a Hilbert space H, what do we mean by the closed subspace of H.

A subspace of a vector space (and a Hilbert space is a vector space) is a nonempty set X such that

  • [itex]x,y\in X~\Rightarrow~x+y\in X[/itex]
  • [itex]x\in X,\alpha\in \mathbb{C}~\Rightarrow~\alpha x\in X[/itex]

A Hilbert space also comes equipped with a norm:

[tex]\|x\|=\sqrt{<x,x>}[/tex]

and a set X is closed under the norm if for all convergent sequences in X it holds that the limit is in X.

A closed subspace is something that is both a subspace and closed.

Also, Does every Hilbert space have an identity? :P.

What do you mean with identity?? It has a 0, which is the additive identity...
 
Thank You.
What do you mean with identity?? It has a 0, which is the additive identity...

I mean 1.
 
LikeMath said:
I mean 1.

That doesn't really help me. What is 1 supposed to mean??
 
micromass said:
That doesn't really help me. What is 1 supposed to mean??

1 is the multiplicative identity
 
LikeMath said:
1 is the multiplicative identity

And since when does a Hilbert space have a multiplication??
 
Oops, thank you, that is why the question is stupid.
 
LikeMath said:
Oops, thank you, that is why the question is stupid.

There are no stupid questions :-p
 

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