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Is there a Bernstein subset $X$ of $\mathbb{R}$ such that no continuous map $f : X → [0,1]$ is surjective ?

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No, if $X$ is a Bernstein set then there is a continuous surjection $X \rightarrow [0,1]$.

To see this, note that there is a continuous surjection $f: \mathbb R \rightarrow [0,1]$ such that the preimage of every point is an uncountable closed set. (For example, $f$ could be the composition of a space filling curve $\mathbb R \rightarrow [0,1]^2$ followed by a projection map $[0,1]^2 \rightarrow [0,1]$.) If $X \subseteq \mathbb R$ is a Bernstein set, then $X$ meets $f^{-1}(x)$ for all $x \in [0,1]$. Hence $f \restriction X$ is a surjection.

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  • $\begingroup$ This is beautiful, Will! $\endgroup$ Commented Mar 13 at 20:33
  • $\begingroup$ Thanks Boaz! It's always fun when space-filling curves make an appearance. $\endgroup$
    – Will Brian
    Commented Mar 13 at 20:37

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