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Let $1<p<\infty$. Recall that a Banach space $X$ is $l_{p}$-saturated if every infinite-dimensional subspace of $X$ contains a subspace isomorphic to $l_{p}$. I have a seemingly stronger notion of $l_{p}$-saturated Banach spaces. We say that a Banach space $X$ is strongly $l_{p}$-saturated if for every infinite-dimensional subspace $M$ and every $\epsilon>0$, there exists an infinite-dimensional subspace $N$ of $M$ such that $N$ is $(1+\epsilon)$-isomorphic to $l_{p}$. I have the following two questions:

Question 1. If a Banach space $X$ is $l_{p}$-saturated, is $X$ strongly $l_{p}$-saturated?

Question 2. Are there other examples of strongly $l_{p}$-saturated spaces besides $l_{p}$? Is $L_{p}$ strongly $l_{p}$-saturated?

Thank you!

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  • $\begingroup$ $L_p$ contains copies of $\ell_2$; it is not $\ell_p$-saturated. $\endgroup$
    – Tomasz Kania
    Dec 8, 2016 at 15:12
  • $\begingroup$ The Lorentz and Garling sequence spaces and certain Orlicz sequence spaces are strongly $\ell_p$-saturated. Probably so are the Lorentz-Orlicz and Musielak-Orlicz sequence spaces. Other spaces which I don't remember off the top of my head but are worth checking, include the $(p,q,\xi)$-Schreier spaces, the James spaces $J_p$ and spaces of the form $(\oplus X_n)_p$ for dim$(X_n)<\infty$. Note that the Lorentz sequence spaces are saturated with $(1+\epsilon)$-complemented copies of $\ell_p$, and probably also some Orlicz. It is an open question whether the same goes for Garling sequence spaces. $\endgroup$
    – Ben W
    Dec 8, 2016 at 15:54
  • $\begingroup$ @BenWallis, Could you check that $(\oplus X_{n})_{p}$ for $dim(X_{n})<\infty$ is strongly $l_{p}$-saturated? I can check that the James $p$-space is strongly $l_{p}$-saturated. $\endgroup$
    – Dongyang Chen
    Dec 9, 2016 at 12:08
  • $\begingroup$ For $(\oplus X_n)_p$, as we may assume $1<p<\infty$, this is just a gliding hump argument. $\endgroup$
    – Ben W
    Dec 9, 2016 at 12:31
  • $\begingroup$ One condition which guarantees that an $\ell_p$ saturated space be strongly $\ell_p$ saturated would be that the space has a Schauder basis $(e_i)_{i=1}^\infty$ (or even just an FDD) such that either $(1)$ for any $n\in\mathbb{N}$, $0=k_0<\ldots <k_n$, and $x_i=\sum_{j=k_{i-1}+1}^{k_i} a_je_j$, $$\|\sum_{i=1}^n x_i\|^p \leqslant \sum_{i=1}^n \|x_i\|^p,$$ or $(2)$ for any $n\in\mathbb{N}$, $0=k_0<\ldots <k_n$, and $x_i=\sum_{j=k_{i-1}+1}^{k_i} a_je_j$, $$\|\sum_{i=1}^n x_i\|^p \geqslant \sum_{i=1}^n \|x_i\|^p.$$ It would also be true if we only assumed such an inequality for skipped blocks. $\endgroup$
    – user114263
    Jul 31, 2018 at 17:55

1 Answer 1

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As for question 1; this is the famous distortion problem. Your question has negative answer already for some renorming of $\ell_p$:

E. Odell, Th. Schlumprecht, The distortion problem of Hilbert space, Geom. Funct. Anal., 3, 201–207.

As for your second question, $L_p$ is not $\ell_p$-saturaded as the Rademachers span a copy of $\ell_2$.

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  • $\begingroup$ Thanks, Ben and Tomek. $L_{p}$ is not $l_{p}$-saturated. But I have another question on $L_{p}$. Given any infinite-dimensional closed subspace $M$ of $L_{p}(1<p<\infty)$ and any $\epsilon>0$. Do there exist an infinite-dimensional closed subspace $N$ of $M$ and $1<q<\infty$ such that $N$ is $(1+\epsilon)$-isomorphic to $l_{q}$? $\endgroup$
    – Dongyang Chen
    Dec 9, 2016 at 9:10
  • $\begingroup$ I'm not completely sure, but my guess is that probably yes. I would look at chapter 6 from the Albiac/Kalton book to see if the isomorphism constants can be made that small. At least for $L_p$, $2\leq p<\infty$, it "should" be doable. $\endgroup$
    – Ben W
    Dec 9, 2016 at 10:48

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