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Let $p\in [1,\infty)\setminus\{2\}$. Suppose $(e_n)$ is a basic sequence in $\ell_p$ (or $L_p$) equivalent to the basis of $\ell_p$ ($L_p$). Is there a subsequence $(e_{n_k})$ such that $[e_{n_k}]$ is complemented?

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Yes, and it is simple fact that is proved in many books, including Albiac-Kalton. This is not an MO level question. Now if you had asked about $L_p$... – Bill Johnson Mar 29 '12 at 17:16
Thank you. Indeed, I am also interested in the $L_p$ case as well so let me modify my question. – Olaf Kummers Mar 29 '12 at 17:43
up vote 2 down vote accepted

The answer is yes also for $L_p$, but I don't know a good book reference. For $2<p<\infty$, this is contained the paper of Kadec and Pelczynski--it is their second dichotomy theorem. Actually, they get that a normalized weakly null sequence has a subsequence that is either equivalent to an orthonormal sequence (in which case its closed span is automatically complemented) or has, for every $\epsilon > 0$, a subsequence that is $1+\epsilon$-equivalent to the unit vector basis for $\ell_p$ and spans a subspace that is $1+\epsilon$-complemented.

For $1\le p <2$, I think the result was pointed out by Pelczynski but I don't know a reference. It follows from arguments like those in Wojtaszczyk's book characterizing weak compactness in $L_1$. You can find an outline of the argument in a paper I wrote with G. Schechtman:

Multiplication operators on L(Lp) and lp-strictly singular operators, J. European Math. Society 10 1105-1119 (2008), which you can download from my home page.

EDIT July 7, 2012: The result above that I attributed to Pelczynski is actually due to Enflo and Rosenthal:

Enflo, Per; Rosenthal, Haskell P. Some results concerning Lp(μ)-spaces. J. Functional Analysis 14 (1973), 325–348.

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