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We can state the axiom of choice ($\mathsf{AC}$) in the following way $$\forall x\Big(\neg(\varnothing\in x)\longrightarrow\exists f\big(f\text{ is a function from $x$ to $\bigcup x$ }\wedge \ \forall y\in x(f(y)\in y)\big)\Big). $$This statement is very clear and explains why we call it the axiom of choice.

There are many many useful theorems that require $\mathsf{AC}$, but many of them do not require its full power. For example, we have the ultrafilter lemma ($\mathsf{UL}$), which is much weaker than $\mathsf{AC}$. We also have dependent choice ($\mathsf{DC}$), which is stronger than $\mathsf{AC}_\omega$ and weaker than $\mathsf{AC}$.

My question is, do we know of any statement equivalent to, for example, the $\mathsf{UL}$ and that maintains the choice style? By "choice style" I mean a statement like "for any set with certain properties there exists a choice function" ($\mathsf{AC}_\omega$ is an example). I am also interested in answers to the previous question but with another (interesting, if possible) statement instead of $\mathsf{UL}$.

My intuition tells me that perhaps it is not a very relevant question because, for example, for any infinite cardinal $\kappa$, there may be a statement $\varphi$ stronger than $\mathsf{AC}_\kappa$ but weaker than $\mathsf{AC}_{\kappa^+}$ and there may not even be a statement "between" $\mathsf{AC}_\kappa$ and $\mathsf{AC}_{\kappa^+}$ with this choice style (or it is very difficult), I don't know.

Yester
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  • What is the "ultrafilter lemma" and how is it weaker than the countable axiom of choice? – bof Nov 30 '23 at 13:37
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    That's what I thought, but what makes you think UL is weaker than the countable axiom of choice? I don't believe countable choice is enough to prove the existence of a nonprincipal ultrafilter on $\omega$, let alone that any filter on any set extends to an ultrafilter. I would say that UL and countable choice are incomparable axioms, neither being stronger or weaker than the other. – bof Nov 30 '23 at 14:02
  • @bof, I had in mind that $\mathsf{UL}$ is weaker but, certainly, I have not been able to find a source that confirms this. Thanks. – Yester Nov 30 '23 at 14:09

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