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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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Class-theoretic sentences that are $\Pi^1_1$ or $\Pi^1_2$
@PaulBlainLevy That's implied by Vopenka's principle, since SCH holds above a strongly compact cardinal.
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Class-theoretic sentences that are $\Pi^1_1$ or $\Pi^1_2$
I'm pretty sure KM proves that for every $X$ there is a satisfaction class for the language expanded by $X.$ It certainly holds for $(V_{\kappa}, V_{\kappa+1}),$ $\kappa$ inaccessible.
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Class-theoretic sentences that are $\Pi^1_1$ or $\Pi^1_2$
To remove ambiguity about what choice, reflection, and large cardinal principles are, it might be easier to work in an ambient model of ZFC and ask about models of the form $(V_{\kappa}, V_{\kappa+1}),$ where $\kappa$ is assumed to have large cardinal properties.
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Dedekind-"finiteness" for arbitrary limit cardinals
ZF + DC + "$\omega_1$ doesn't inject into $\mathbb{R}$" has consistency strength an inaccessible since it implies $\omega_1$ is inaccessible in $L.$ In particular, it's not a theorem of "all sets of reals have property of Baire."
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Dedekind-"finiteness" for arbitrary limit cardinals
It's consistent with ZF that $\aleph(\mathbb{R})=\aleph_{\omega}.$ Ref: "A model of Z-F set theory with $\aleph(2^{\omega}) = \aleph_{\omega}$" by Derrick and Drake.
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Is the supremum of L-definable cardinals silver-indiscernible
@Reflecting_Ordinal $i_0$ is the sup of countable ordinals which are definable in $L$ from $V$-cardinals (equivalently, from $\omega_n^V$'s).
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Can a Vitali set be Lebesgue measurable? (ZF)
@EmilJeřábek I got confirmation from Gao that it's a ZF theorem.
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Can a Vitali set be Lebesgue measurable? (ZF)
I think their proof yields an explicit construction in this case, but I'll email them to check.
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Is "There exists an unbounded non-measurable set but no bounded non-measurable set" consistent with $\mathsf{ZF}$?
This answer has been significantly overhauled. The above discussion refers to a previous argument which has been generalized to the (3) $\rightarrow$ (4) in the current answer.
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Is "There exists an unbounded non-measurable set but no bounded non-measurable set" consistent with $\mathsf{ZF}$?
This is a significant overhaul, proving a theorem which immediately (and unambiguously) resolves both of the questions.
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Is "There exists an unbounded non-measurable set but no bounded non-measurable set" consistent with $\mathsf{ZF}$?
My comment was referring to my pre-edit definition, which isn’t even closed under complements.
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Is "There exists an unbounded non-measurable set but no bounded non-measurable set" consistent with $\mathsf{ZF}$?
@EmilJeřábek Suppose $X$ is measurable against bounded intervals. Assume $Y$ has finite outer measure (otherwise trivial). Fix an open cover $C$ of $Y$ of measure less than $\lambda^*(Y)+\epsilon.$ Finitely many intervals $\{I_k\}_1^n$ in $C$ contain all but $\frac{\epsilon}{2}$ of the measure. For each $k \le n,$ choose covers $I_k \cap X \subset C_{1, k}$ and $I_k \setminus X \subset C_{2, k}$ such that $\lambda(C_{1,k})+\lambda(C_{2,k})<\lambda(I_k)+\frac{\epsilon}{2n}.$ The rest is clear.
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Is "There exists an unbounded non-measurable set but no bounded non-measurable set" consistent with $\mathsf{ZF}$?
@FrançoisG.Dorais I think your earlier suggestion is reasonable. That if $\mathbb{R}$ is a countable union of countable sets, then there is no total isometry-invariant finitely additive probability measure on $\mathcal{P}([0,1])$ which has the Lebesgue density property.
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Is "There exists an unbounded non-measurable set but no bounded non-measurable set" consistent with $\mathsf{ZF}$?
@FrançoisG.Dorais In ZF, "Lebesgue measure" is a finitely additive measure with the definition given above. The claim is that if $\mathbb{R}$ is a countable union of countable sets, there is a set which is not Lebesgue measurable, giving an affirmative answer to question 1.
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