Take the 2-minute tour ×
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.

I was trying to find papers and articles about non-contractive continuous projections in $\ell_1(S)$ where $S$ is an arbitrary set. If it is not studied yet, I would like to know results for the case $S=\mathbb{N}$.

I've found one quite general condition for the closed linear subspace to be image of a continuous projection. Such a subspace must be the closure of the linear span of so-called relatively disjoint vectors. This subspaces gives us explicit examples of projections with norm greater than 1. For details see the paper of H. P. Rosenthal On relatively disjoint families of measures, with some applications to Banach space theory.

As a special case we can get subspaces that are the closure of the linear span of disjointly supported vectors. These subspaces give us examples of norm one projections. Moreover, only such subspaces are give rise to norm one projections. For details see the survey by Beata Randrianantoanina Norm one projections in Banach spaces.

Thus, for projections of norm 1 we have a complete description. For the rest quite a big source of examples. In the first mentioned paper the author states that he doesn't know any other examples of continuous projections in $\ell_1(S)$ that are not generated by some relatively disjoint family of vectors.

So, could someone give me a reference where I can read about other examples of projections in $\ell_1(S)$, or may be their complete characterization?

Also I will be grateful if you give me some explicit examples of discontinuous projections in $\ell_1(S)$.

The same question on mathoverflow.net.

share|improve this question
4  
If you put a 500 bounty on this its chances of getting answered will sky rocket. –  Ragib Zaman Jun 10 '12 at 15:00
    
Are you interested in (1) subspaces which don't admit continuous projections [namely, uncomplemented subspaces], or (2) in discontinuous projections themselves? The examples of (2) can be constructed even in a Hilbert space, where all subspaces are complemented. –  user31373 Jun 16 '12 at 18:56
    
I'm interested in quite explicit description of non-contractive projections. I've will have it, of course, I'll get examples of discontinuous projections. Examples of uncomplemented subspaces will be helpful too. –  no identity Jun 16 '12 at 19:30
4  
@Yemon: I don't understand your comment. There is no subspace of $\ell^1$ isomorphic to $\ell^2$ by Pitt's theorem. A closed infinite-dimensional subspace of $\ell^p$ cannot be isomorphic to a closed subspace of $\ell^q$ if $1 \leq p \neq q \lt \infty$ since the operator in one direction would have to be compact. –  t.b. Jul 2 '12 at 12:14
3  
@francis-jamet: It's impossible to prove that there is a discontinuous linear map without invoking some strong form of choice -- "you can't write down a discontinuous linear map defined on all of a Banach space". For instance, it follows from the axiom of determinacy that every linear map on a separable Banach space is continuous. –  t.b. Jul 2 '12 at 14:45

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.