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Let $X$ be a compact Hausdorff space and $C(X)$ its ring of continuous (real valued) functions. Then it's well known that points of $X$ are in bijection with maximal ideals of $C(X)$, but the spectrum as you might compute in algebraic geometry is quite wild since there are many nonmaximal primes which are very hard to understand. See here or here for instance, or here for a very surprising result that there's often a $[0,\infty)$-indexed chain of prime ideals!

But it seems like all of these results rely on the axiom of choice, often via either ultrafilters or Zorn's lemma... Which makes me wonder the question in the title:

Is it consistent with ZF that every prime ideal of $C(X)$ is maximal?

This would be another way of saying that the spectrum of $C(X)$ really "should be" $X$. Usually we do this by working with the max spectrum, but it feels more algebro-geometrically natural to work with the prime spectrum and restrict attention to "tame primes" which can be constructed without choice.

Thanks in advance!

rschwieb
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  • How do you know that in ZF, the ideals corresponding to different points are distinct? From what I read, Urysohn lemma doesn't need to hold in ZF, so there might not be enough functions. – Jakobian Apr 03 '25 at 23:28

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The obvious answer is yes, since any finite space $X$ is an example. But you might want to assume that $X$ is infinite, so I've prepared some proofs below. In the first part I describe the difficulties of the case of ZF, and in the second part I explain how there's no such infinite space in ZFC.


A Tychonoff space $X$ is called a $P$-space when every $G_\delta$-set is open.

I've replaced compact Hausdorff with compact Tychonoff, since I suspect that compact Hausdorff spaces might not have enough continuous functions in ZF, and that it's possible that maximal ideals are not even distinct. This is because ZF with countable choice is not enough to prove Urysohn's lemma.

  • Let $X$ be a compact Tychonoff space. Then every prime ideal $P\subseteq C(X)$ is contained in a unique maximal ideal.

Proof: For $f\in C(X)$ let $Z(f) = \{x\in X : f(x) = 0\}$, $M_p = \{f\in C(X) : p\in Z(f)\}$ and $O_p = \{f\in C(X) : p\in\text{int}(Z(f))\}$. If $P\subseteq C(X)$ is a prime ideal, there is $p\in X$ such that $P\subseteq M_p$. Let $f\in O_p$, and find $g$ such that $g(x) = 0$ for $x\notin\text{int}(Z(f))$ and $g(p) = 1$. Then $fg = 0 \in P$ and $g\notin M_p$ so that $f\in P$. It follows that $O_p\subseteq P$. Suppose that $q\neq p$. Find $f\in C(X)$ such that $f(q)\neq 0$ and $f$ vanishes on a neighbourhood of $p$. Then $f\in O_p\setminus M_q$, and so $P\not\subseteq M_q$. $\square$

  • Let $X$ be a compact Tychonoff space and every $O_p$ is an intersection of prime ideals. Then $X$ is a $P$-space iff every prime ideal of $C(X)$ is maximal.

Proof: If $X$ is a $P$-space, and $p\in X$, let $f\in M_p$ then $Z(f)$ is a $G_\delta$-set, and so open, and so $f\in O_p$. It follows that $O_p = M_p$. Now if $P$ is a prime ideal, it follows from the previous proof that there is $p\in X$ with $O_p\subseteq P\subseteq M_p$, and so $P = M_p$. Now conversely suppose every prime ideal is maximal. If $O_p\subsetneq M_p$, let $f\in M_p\setminus O_p$. Find a prime ideal $f\notin P$ with $O_p\subseteq P\subsetneq M_p$, then $P$ is prime but not maximal. $\square$

Now the issue here is that I don't know how to get rid of the assumption that $O_p$ is a intersection of prime ideals without assuming AC. Normally I'd prove it by the following argument: Since $f^n\in O_p$ implies $f\in O_p$, there exists a prime ideal $P$ with $O_p\subseteq P$ and $f\notin P$, but this uses Zorn's lemma.

  • A compact Tychonoff $P$-space $X$ is Dedekind-finite.

Proof: Suppose otherwise, and take countably infinite $A\subseteq X$. Then $A$ is a closed copy of $\mathbb{N}$, which is impossible since that would imply it's compact. $\square$


In ZFC the situation is much easier, and arguments don't hinge on compactness:

  1. A Tychonoff space $X$ is a $P$-space iff every prime ideal of $C(X)$ is maximal.

  2. A Tychonoff space $X$ is a compact $P$-space iff $X$ is finite.

These results are in Rings of continuous functions by Gillman and Jerison, and are proven by essentially using the arguments laid out above.

Jakobian
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    Given that this is a question over ZF, is choice used in either the classification of compact $P$-spaces or the characterization of $P$-spaces? – Noah Schweber Apr 03 '25 at 22:59
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    @NoahSchweber I've went through the proofs of those, and I've replaced the equivalence of compact $P$-space and finite space, with compact $P$-spaces being Dedekind-finite. Also the characterization of $P$-spaces has a problematic step which uses Zorn's lemma – Jakobian Apr 04 '25 at 00:30