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YCor
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Suppose $X$ is a separable infinite dimensional Banach space, $E$ a finite dimensional subspace which is $c$-complemented in $X$. Is the following true?

For any $\epsilon>0$$\varepsilon>0$, one can find $x\notin E$ such that $span\{E,x\}$$\mathrm{span}\{E,x\}$ is $c+\epsilon$ complemented$(c+\varepsilon)$-complemented in $X$.

This is obviously true in Hilbert spaces, but not sure in general Banach spaces. Would reflexivity make a difference?

Suppose $X$ is a separable infinite dimensional Banach space, $E$ a finite dimensional subspace which is $c$-complemented in $X$. Is the following true?

For any $\epsilon>0$, one can find $x\notin E$ such that $span\{E,x\}$ is $c+\epsilon$ complemented in $X$.

This is obviously true in Hilbert spaces, but not sure in general Banach spaces. Would reflexivity make a difference?

Suppose $X$ is a separable infinite dimensional Banach space, $E$ a finite dimensional subspace which is $c$-complemented in $X$. Is the following true?

For any $\varepsilon>0$, one can find $x\notin E$ such that $\mathrm{span}\{E,x\}$ is $(c+\varepsilon)$-complemented in $X$.

This is obviously true in Hilbert spaces, but not sure in general Banach spaces. Would reflexivity make a difference?

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Markus
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Suppose $X$ is a separable infinite dimensional Banach space, $E$ a finite dimensional subspace which is $c$-complemented in $X$. Is the following true?

For any $\epsilon>0$, one can find $x\notin E$ such that $span\{E,x\}$ is $c+\epsilon$ complemented in $X$.

This is obviously true in Hilbert spaces, but not sure in general Banach spaces. Would reflexivity make a difference?

Suppose $X$ is a separable infinite dimensional Banach space, $E$ a finite dimensional subspace which is $c$-complemented in $X$. Is the following true?

For any $\epsilon>0$, one can find $x\notin E$ such that $span\{E,x\}$ is $c+\epsilon$ complemented.

This is obviously true in Hilbert spaces, but not sure in general Banach spaces. Would reflexivity make a difference?

Suppose $X$ is a separable infinite dimensional Banach space, $E$ a finite dimensional subspace which is $c$-complemented in $X$. Is the following true?

For any $\epsilon>0$, one can find $x\notin E$ such that $span\{E,x\}$ is $c+\epsilon$ complemented in $X$.

This is obviously true in Hilbert spaces, but not sure in general Banach spaces. Would reflexivity make a difference?

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Markus
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  • 13

Complementability of finite dimensional subspaces

Suppose $X$ is a separable infinite dimensional Banach space, $E$ a finite dimensional subspace which is $c$-complemented in $X$. Is the following true?

For any $\epsilon>0$, one can find $x\notin E$ such that $span\{E,x\}$ is $c+\epsilon$ complemented.

This is obviously true in Hilbert spaces, but not sure in general Banach spaces. Would reflexivity make a difference?