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Elliot Glazer's user avatar
Elliot Glazer's user avatar
Elliot Glazer's user avatar
Elliot Glazer
  • Member for 7 years, 7 months
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How much of mathematical General Relativity depends on the Axiom of Choice?
The listed facts are provable in ZF. Do you know of any natural ZF theorems of analysis that need countable choice when working in these sorts of subsystems? Btw since I’m not the only commenter on your answer you need to @ me for me to be notified if you respond.
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How much of mathematical General Relativity depends on the Axiom of Choice?
I see. Sifting through your papers, I think the arguments justifying my previous comment are formalizable in $Z_2^{\Omega},$ and that this theory is a conservative extension of $Z_2$ which has third-order objects in its ontology. Is my understanding correct?
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What is lost in General Relativity without Hahn-Banach axiom in the ZF+HB set theory?
@JamesHanson Even full HB does not imply there is an np ultrafilter on $\mathbb{N},$ see jstor.org/stable/2272118 Thm 2. Incidentally, full UFL can be decomposed into the ordering principle (OP) and compactness for ordered languages (CfO), the latter implying "HB-for-sep-normed-spaces." I've been investigating the conjecture that OP and CfO are separately (but not together) consistent with a total, isometry-invariant extension of Lebesgue measure on $\mathbb{R}^n.$
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What is lost in General Relativity without Hahn-Banach axiom in the ZF+HB set theory?
The chain in your PS isn't quite right. HB does not imply ADC. In fact, UFL holds and countable choice fails in the Cohen model with an infinite Dedekind finite set of reals.
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How much of mathematical General Relativity depends on the Axiom of Choice?
I'm confused about your (A) vs (B) dichotomy. Bounded variation functions and semi-continuous functions are in Baire class 1, and Baire class 2 is the same as effective Baire class 2. These are all provable in much less than ZF (say, $Z_2$).
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What is lost in General Relativity without Hahn-Banach axiom in the ZF+HB set theory?
@JamesHanson Sorry my previous comment was just following up on whether Riemannian manifolds are second countable. I haven’t checked if the Prufer manifold can be paracompact.
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What is lost in General Relativity without Hahn-Banach axiom in the ZF+HB set theory?
For a connected Riemannian manifold, I think this works: fix $p,$ an $(n-1)$-sphere $S$ centered at $p,$ and a countable dense $X \subset S.$ For points $q$ of rational distance from $p$ via a geodesic through a point in $X,$ take the balls $B(q, 1/n).$
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What is lost in General Relativity without Hahn-Banach axiom in the ZF+HB set theory?
@JamesHanson A second countable manifold is a Polish space, so Shoenfield applies and you get basically the whole ZFC theory, include embeddings into $\mathbb{R}^n.$ I don’t know if being connected metrizable or even a connected Riemannian manifold is enough to prove second countability.
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What is lost in General Relativity without Hahn-Banach axiom in the ZF+HB set theory?
@JamesHanson They’re inequivalent. It’s a ZF theorem that the long line is not metrizable (if an ordinal $\alpha$ has a metric, one can can transfinitely construct a canonical enumeration for each $\beta \le \alpha$).
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What is lost in General Relativity without Hahn-Banach axiom in the ZF+HB set theory?
@JamesHanson But without choice, it’s consistent that the long line is paracompact (this is equivalent to $\omega_1$ being singular). I bet it’s consistent with ZF that all manifolds are paracompact.
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Ideal-like filter on a ring not generated by ring ideals
@IlyaBogdanov Sorry to repeatedly ping you, but I've now proven the claim for Noetherian rings.
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Ideal-like filter on a ring not generated by ring ideals
I added a counterexample for a general commutative ring.
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Ideal-like filter on a ring not generated by ring ideals
Provided a counterexample to the most general claim.
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Ideal-like filter on a ring not generated by ring ideals
I've generalized the claim to Dedekind domains. I don't think this argument can be pushed much further, but it's plausible the claim holds for arbitrary Noetherian domains.
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Ideal-like filter on a ring not generated by ring ideals
I generalized the result from PID's to Dedekind domains.
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