Banach Space Problem: Proving Subspaces Contain e-Orthogonal Elements

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SUMMARY

The discussion centers on the Banach space problem regarding e-orthogonal elements in closed subspaces. It establishes that for any positive epsilon (e) less than 1, proper subspaces of a closed subspace M of a Banach space E contain e-orthogonal elements. The Riesz Lemma is referenced as a foundational result that supports this conclusion. The discussion clarifies that the only element in M that is e-orthogonal is the zero vector, emphasizing the distinction between elements in M and those in E.

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Homework Statement



Let E be a Banach space and let M be a closed subspace of E. A
vector x in E is called e-orthogonal to M if for all y in M the following inequality holds: ||x+y||>= (1- e)||x||.


Prove that for each e>0 any proper subspace of M contains e-orthogonal
elements.


The Attempt at a Solution



Well clearly 0 is always such an element but how to find more elements?
 
Last edited:
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What is e? A positive number less than 1? And did you really mean to say that M contains e-orthogonal elements? Because the only such possible element is 0. Proof: if x is in M and x is e-orthogonal to M, then ||x||(1-e) <= ||x+(-x)||=0.

Edit: And if you meant to say "E contains elements which are e-orthogonal to M", then this result is known as (or at least immediately follows from) the Riesz Lemma.
 
Last edited:

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