Tell me more ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Riesz's lemma state that

If $Y$ is a proper, closed subspace of a normed space $X$, Then for any $\epsilon>0$, there exists $x$ in the closed unit ball of $X$ such that $d(x,Y)>1-\epsilon$.

My question is, can we state something stronger--does there exists $x$ in the closed unit ball whose distance to $Y$ equals 1?

share|improve this question
1  
From Diestel's Sequences and Series in Banach Spaces, page 6: "R. C. James (1964) proved that a Banach space $X$ is reflexive if and only if each $x^*\in X^*$ achieves its norm on $B_X$. Using this, one can establish the following: For a Banach space $X$ to have the property that given a proper closed linear subspace $Y$ of $X$ there exists an $x$ of norm-one such that $d(x,Y)\ge 1$ it is necessary and sufficient that $X$ be reflexive." – David Mitra Feb 6 at 16:24
A specific counterexample to your claim is: Let $X$ be the subspace of $C[0,1]$ consisting of the members of $C[0,1]$ that vanish at $0$. Let $Y$ be the subspace of $X$ consisting of those functions $f$ for which $\int_0^1 f(x)\,dx=0$. Then there is no $x\in S_X$ with $d(x,Y)\ge1$ (and thus no $x\in B_x$ with $d(x,Y)\ge1$). – David Mitra Feb 6 at 16:42
Thank you very much. I followed your lines and got it. – Montez Feb 6 at 17:52

Know someone who can answer? Share a link to this question via email, Google+, Twitter, or Facebook.

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.