Let $X$ be a completely regular (Tychonoff) topological space. It is known that if $\mathscr F\subseteq C(X,[0,1])$ separates points and closed sets (that is, for every closed set $E\subseteq X$ and $x\in X\setminus E$, $\exists f\in\mathscr F$ such that $f(x)\notin\operatorname{cl}[f(E)]$), then $X$ can be densely embedded into a compact Hausdorff space. Namely, considering the compact Hausdorff space $[0,1]^{\mathscr F}$ (with the product topology), there exists an embedding $e:X\to e(X)\subseteq [0,1]^{\mathscr F}$ such that the projections satisfy $\pi_{f}(e(x))=f(x)$ for all $x\in X$ and $f\in\mathscr F$. Defining $Y$ to be the closure of $e(X)$ in $[0,1]^{\mathscr F}$, $(Y,e)$ is called a Hausdorff compatification of $X$ associated to $\mathscr F$.

Now let $BC(X)$ denote the space of bounded continuous complex-valued functions on $X$. An algebra $\mathscr A\subseteq BC(X)$ (that is, a vector space that contains also the product of any two of its members) is called completely regular if (i) $\mathscr A$ is closed; (ii) $1\in\mathscr A$ ($1$ denotes the constant function); (iii) $\mathscr A\cap C(X,[0,1])$ separates points and closed sets.

It can be shown that if $(Y,e)$ is a Hausdorff compatification of $X$, then $\mathscr A_Y\equiv\{F\circ e\,|\,F\in C(Y)\}$ is a completely regular subalgebra of $BC(X)$. Moreover, if $(Y,e)$ and $(Y',e')$ are two Hausdorff compatification that give rise to the same completely regular algebra $\mathscr A_Y=\mathscr A_{Y'}$, then $(Y,e)$ and $(Y',e')$ must be homeomorphic. If one is willing to identify homeomorphic compactifications, it follows that the map $(Y,e)\mapsto \mathscr A_Y$ from Hausdorff compatifications of $X$ to completely regular subalgebras of $BC(X)$ is injective.

What I want to show is that this map is actually also surjective. That is,

Conjecture:$\quad$ Any given completely regular subalgebra of $\mathscr A\subseteq BC(X)$ is equal to $\mathscr A_Y$ for some Hausdorff compactification $(Y,e)$.

The following fact is known:

  • If $(Y,e)$ is the Hausdorff compatification of $X$ associated to some $\mathscr F\subseteq C(X,[0,1])$, then $\mathscr A_Y$ is the smallest closed subalgebra of $BC(X)$ that contains $\mathscr F$ and the constant function $1$.

Hence, given a completely regular algebra $\mathscr A\subseteq BC(X)$, in order to construct a Hausdorff compatification $(Y,e)$ for which $\mathscr A=\mathscr A_Y$, an obvious candidate would be to take $\mathscr F=\mathscr A\cap C(X,[0,1])$. What I am unable to show is that $\mathscr A$ is really the smallest closed subalgebra that contains its intersection with $C(X,[0,1])$. If this conjecture is not true (it may be the case if one does not require $\mathscr A$ to separate points and closed sets—see here), can one at least find such an $\mathscr F$ that $\mathscr A\cap C(X,[0,1])\subseteq \mathscr F\subseteq C(X,[0,1])$ and the resulting Hausdorff compactification $(Y,e)$ gives exactly $\mathscr A_Y=\mathscr A$? (Example: if $\mathscr F=C(X,[0,1])$, then the associated compactification is the Stone–Čech compactification $\beta X$ and $\mathscr A_{\beta X}=BC(X)$, but I need a “smaller” compactification that gives exactly $\mathscr A$.)

Any hints and comments would be greatly appreciated.

UPDATE: Note that for any compactification $(Y,e)$, $\mathscr A_Y$ is closed under complex conjugation. Indeed, if $f\in \mathscr A_Y$, then $f=F\circ e$ for some $F\in C(Y)$. Since $\overline F\in C(Y)$, it follows that $\overline f=\overline F\circ e\in \mathscr A_Y$. Hence, in order for the conjecture above to be true, it must be the case that every completely regular subalgebra of $BC(X)$ is closed under complex conjugation. I am wondering whether this happens to be the case.


The conjecture is false. I also asked this question on Mathoverflow and Eric Wofsey provided a counterexample.


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