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Suppose $X$ is a closed subspace of $c_0$ with an unconditional basis and suppose also that it is a quotient of $c_0$. Is $X$ also a complemented subspace of $c_0$?

An affirmative answer implies that $X$ is isomorphic to $c_0$. So, in particular, is every closed subspace of $c_0$ that which is a quotient of $c_0$, isomorphic to $c_0$?

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No. $(\sum_{n=1}^\infty \ell_2^n)_{c_0}$ is a quotient of $c_0$.

As I say to my students, "You know this; the problem is to figure out why you know this". :)

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    $\begingroup$ +1 for the comment to students! $\endgroup$ Commented Jun 17, 2022 at 20:02

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