This is a follow up to: Complemented subspace constructed from finite pieces

Suppose $Y=\overline{\cup E_n}$ is a closed subspace of a separable Banach space X, where each $E_n$ is a $n$-dimensional subspace, $K$-complemented in $X$, and for any $n$, $E_n\subseteq E_{n+1}$. Can one conclude that $Y$ is complemented in $X$?

In light of the answer to the previous question, a related question would be the following:

Is $c_0$ complemented in every separable subspace of $l_\infty$ that contains it. I suspect the answer is no, but cannot think of a counterexample.


1 Answer 1


The answer to your question in the last paragraph is "Yes", it is a contents of the well-known Sobczyk Theorem, see Lindenstrauss-Tzafriri, Classical Banach spaces, vol. I.

The answer to the first question is still negative. You can find an example of this type in W.B. Johnson, J. Lindenstrauss, Examples of L1 spaces, Ark. Mat. 18 (1980), no. 1, 101–106.

The answer is positive if the space is reflexive - you can consider the weak limit of the projections.

Added on January 5, 2018. Many other examples giving a negative answer to the first question can be constructed combining the result of Zippin [Israel J. Math. 26 (1977), no. 3-4, 372–387] stating that any separable infinite-dimensional Banach space which is not isomorphic to $c_0$ can be embedded into a separable Banach space as an uncomplemented subspace, and the well-known fact that a Banach space can represented as a closure of the union of increasing sequence of finite-dimensional subspaces which are uniformly close to $\ell_\infty^n$ without being isomorphic to $c_0$ (see, e.g., very exotic examples in Bourgain, Pisier [Bol. Soc. Brasil. Mat. 14 (1983), no. 2, 109–123].)

  • $\begingroup$ In the case that $X$ is reflexive (but perhaps non-separable), how can we make sure that a weak limit point $P$ of the projections $P_n:X\to E_n$ is a projection onto $Y$? The range of $P$ might contain $Y$ properly. Am I missing something simple? $\endgroup$
    – Onur Oktay
    Commented Nov 29, 2021 at 12:59

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