Does every Hilbert space have an identity?

In summary, the conversation discusses the concept of a closed subspace in a Hilbert space and whether or not every Hilbert space has an identity element. The definition of a subspace and a closed subspace are given, along with the norm of a Hilbert space. The conversation concludes with clarifying what is meant by an identity element and acknowledging that the question may have been unclear.
  • #1
LikeMath
62
0
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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  • #2
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...
 
  • #3
Thank You.
What do you mean with identity?? It has a 0, which is the additive identity...

I mean 1.
 
  • #4
LikeMath said:
I mean 1.

That doesn't really help me. What is 1 supposed to mean??
 
  • #5
micromass said:
That doesn't really help me. What is 1 supposed to mean??

1 is the multiplicative identity
 
  • #6
LikeMath said:
1 is the multiplicative identity

And since when does a Hilbert space have a multiplication??
 
  • #7
Oops, thank you, that is why the question is stupid.
 
  • #8
LikeMath said:
Oops, thank you, that is why the question is stupid.

There are no stupid questions :tongue2:
 

1. What is a Hilbert space?

A Hilbert space is a mathematical concept in functional analysis that refers to an infinite-dimensional vector space with a defined inner product. It is named after the German mathematician David Hilbert and is widely used in fields such as physics, engineering, and mathematics.

2. What does it mean for a Hilbert space to have an identity?

In mathematics, an identity element is an element that, when combined with another element using a specific operation, leaves the other element unchanged. In the context of a Hilbert space, having an identity means that there exists an element that, when used in an inner product with any other element, does not change the other element.

3. Does every Hilbert space have an identity?

Yes, every Hilbert space has an identity element. This is because the concept of an identity element is a fundamental property of Hilbert spaces and is defined as part of the structure of a Hilbert space.

4. How is the identity element of a Hilbert space represented?

The identity element of a Hilbert space is typically represented by the number 1, as it is the multiplicative identity element in most Hilbert spaces. In some cases, it may also be represented by the zero vector, as it is the additive identity element in many Hilbert spaces.

5. Are there any exceptions to the rule that every Hilbert space has an identity?

No, there are no exceptions to this rule. The concept of an identity element is a fundamental property of Hilbert spaces, and it is a necessary component for the proper functioning of a Hilbert space. Therefore, all Hilbert spaces must have an identity element.

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