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Let $X$ and $Y$ be compact Hausdorff spaces and let $\varphi:X\to Y$ be continuous with a property that if $A$ is a nowhere dense zero-set in $Y$, then $\varphi^{-1}(A)$ is nowhere dense in $X$. Let $Z=\varphi(X)$.

Does $\varphi$ still have the analogous property as a map into $Z$?

Note that the condition implies that the set $J_{Z}$ of $f\in C(Y)$ which vanish on $Z$ is a $\sigma$-ideal in $C(Y)$, which means that it contains all existing supremums of countable sets in $J_{Z}$. Such $Z$'s could be considered countable analogues of regular closed sets, because $Z$ is regular if and only if $J_{Z}$ contains existing supremums of all sets.

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    $\begingroup$ Do you mean $Z=\varphi(X)=\mathrm{im}(\varphi)$? Should the title read "surjectification"? $\endgroup$ Commented Feb 10, 2022 at 19:40
  • $\begingroup$ @AlexKruckman yes, thank you! $\endgroup$
    – erz
    Commented Feb 10, 2022 at 20:29
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    $\begingroup$ Does "zero-set" mean that $A$ equals $f^{-1}(0)$ for some continuous function $f : X \to Y$? \\ Also see mathoverflow.net/questions/405778/… for names for your property. $\endgroup$
    – LSpice
    Commented Feb 10, 2022 at 20:31
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    $\begingroup$ @LSpice indeed. Thank you for the link! $\endgroup$
    – erz
    Commented Feb 10, 2022 at 22:23

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The answer here is negative: for $Y$ take the remainder $\beta\omega\setminus\omega$ of the Stone-Cech remander of the discrete space $\omega$ of finite ordinals. In the space $Y$ take any countable discrete subspace $D$ and let $Z$ be the closure of $D$. Since $Y$ has no isolated points, the space $D$ is nowhere dense in $Y$ and so is its closure $Z$. Since $D$ is countable and discrete in the compact space $Z$, the remainder $R=Z\setminus D$ is a nonempty functionally closed nowhere dense set in $Z$. Consider the space $X=(Z\times\{0\})\cup (R\times\{1\})$ and the natural projection $\varphi:X\to Z\subseteq Y$.

Observe that the set $R$ is functionally closed and nowhere dense in $Z$ and its preimage $\varphi^{-1}[R]$ contains the nonempty clopen subset $R\times\{1\}$ of $X$.

On the other hand, each nonempty $G_\delta$-subset of the space $Y=\beta\omega\setminus\omega$ has nonempty interior in $Y$. So, $Y$ contains no functionally closed nowhere dense subsets and hence the function $\varphi:X\to Y$ has the desired property: for every nowhere dense functionally closed set $A$ in $Y$ the preimage $\varphi^{-1}(A)$ has any desired property, in particular is nowhere dense in $X$.

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  • $\begingroup$ Why is $\{z\}$ a zeroset in $Z$? $\endgroup$
    – erz
    Commented Feb 10, 2022 at 23:32
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    $\begingroup$ @erz Indeed, $\{z\}$ is not necessarily a zeroset, but it can be replaced by the nowhere dense zeroset $Z\setminus D$ (which I did in the revised version of the answer). $\endgroup$ Commented Feb 11, 2022 at 11:26
  • $\begingroup$ thank you! (again) $\endgroup$
    – erz
    Commented Feb 11, 2022 at 11:36
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    $\begingroup$ I've sent you an email. This time i used my university address, so hopefully the message won't be lost. Sorry for the late reply, and thank you for the invitation! $\endgroup$
    – erz
    Commented Feb 21, 2022 at 4:34

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