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This tag is for questions about proving that some statement is independent from a theory, meaning it is neither provable nor refutable from that theory. Common examples are the continuum hypothesis from the axioms of ZFC, and the axiom of choice from the axioms of ZF.

7 votes
3 answers
431 views

Dedekind-"finiteness" for arbitrary limit cardinals

In $\mathbf{ZF}$, it is possible for a set $A$ to be infinite but not to admit a countable set. In other words, for any $\alpha\in\omega$, there is an injection from $\alpha$ into $A$, but there is no …
Hannes Jakob's user avatar
  • 1,799
5 votes
1 answer
509 views

Smallest size of a non-measurable set of reals

The question is pretty much the title. I'm wondering if anything is known about the smallest size $\kappa$ of a non-measurable subset of the real numbers (regarding the Lebesgue measure). Since we hav …
Hannes Jakob's user avatar
  • 1,799
5 votes
0 answers
211 views

Questions about very fat sets

If $\kappa$ is a regular uncountable cardinal, we call a set $S\subseteq\kappa$ fat if for every $\alpha<\kappa$ and every club $C\subseteq\kappa$, there is a closed subset of $S\cap C$ of ordertype $ …
Hannes Jakob's user avatar
  • 1,799
9 votes
1 answer
474 views

Relationship between AC, WO, and Zorn's lemma in ZF-Powerset

In regular ZF, AC, WO, and Zorn's Lemma are equivalent, but every proof I know (of the implication AC -> WO and AC -> Zorn) uses the axiom of choice on the powerset of X (where X is the Set which is t …
Hannes Jakob's user avatar
  • 1,799