Question about the Bolzano Weierstrass analogue in Hilbert spaces

In summary, the Bolzano-Weierstrass analogue in Hilbert spaces is a theorem that states every bounded sequence in a Hilbert space has a convergent subsequence, similar to the Bolzano-Weierstrass theorem in real analysis. It is important for analyzing sequences in Hilbert spaces and is proved using the properties of Hilbert spaces. It has been extended to other types of spaces and has applications in mathematics and physics, such as in the spectral theorem and the study of Fourier series.
  • #1
I<3Gauss
14
0
While reading a proof on the closure of the span of finite number vectors in a hilbert space with respect to the norm induced topology, I became stumped on a particular step of the proof using the Bolzano Weierstrass theorem.

For finite dimensional vector spaces, Bolzano Weierstrass states that:

"If ||an||<=M, then an contains a convergent subsequence (wrt whatever metric is defined on this vector space)"

Is it safe to assume that the above theorem extends to infinite dimensional vectors spaces (Hilbert spaces)?

Thanks!
 
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  • #2
No, consider the sequence

(1,0,0,0,...)
(0,1,0,0,...)
(0,0,1,0,...)
...

in [itex]\ell^2[/itex].
 
  • #3
Thanks for the really simple example!
 

Related to Question about the Bolzano Weierstrass analogue in Hilbert spaces

1. What is the Bolzano-Weierstrass analogue in Hilbert spaces?

The Bolzano-Weierstrass analogue in Hilbert spaces is a theorem in functional analysis that states that every bounded sequence in a Hilbert space has a convergent subsequence. This is similar to the Bolzano-Weierstrass theorem in real analysis, which states that every bounded sequence in a Euclidean space has a convergent subsequence.

2. Why is the Bolzano-Weierstrass analogue important?

The Bolzano-Weierstrass analogue is important because it provides a powerful tool for analyzing sequences in Hilbert spaces. It allows us to prove the convergence of sequences without knowing the limit beforehand, making it useful in a variety of mathematical applications.

3. How is the Bolzano-Weierstrass analogue proved in Hilbert spaces?

The proof of the Bolzano-Weierstrass analogue in Hilbert spaces uses the properties of Hilbert spaces, such as completeness and orthogonality. It typically involves constructing a convergent subsequence from a given bounded sequence by using the Cauchy-Schwarz inequality and the Pythagorean theorem.

4. Can the Bolzano-Weierstrass analogue be extended to other types of spaces?

Yes, the Bolzano-Weierstrass analogue has been extended to other types of spaces, such as Banach spaces and metric spaces. These extensions may have slightly different formulations, but they all share the same underlying idea of a bounded sequence having a convergent subsequence.

5. What are the applications of the Bolzano-Weierstrass analogue in Hilbert spaces?

The Bolzano-Weierstrass analogue has numerous applications in mathematics and physics. It is used in the proof of the spectral theorem, which has applications in quantum mechanics. It is also used in the study of Fourier series and in the convergence of numerical methods. Additionally, it has applications in optimization, control theory, and signal processing.

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