Skip to main content
Asaf Karagila's user avatar
Asaf Karagila's user avatar
Asaf Karagila's user avatar
Asaf Karagila
Moderator
  • Member for 14 years, 5 months
  • Last seen this week
comment
Reference request: choiceless cardinality quantifiers
Not so much, I don't think. But that's a good question!
comment
Models of ZF intermediate between a model of ZFC and a generic extension
@Lorenzo Send me an email. This question has a long answer.
revised
Loading…
comment
How much Dependent Choice is provable in $Z_2$? And what about Projective Determinacy?
Thanks, Ali! I've fixed the link to the preprint in the last paragraph, it somehow ended up mangled.
Loading…
comment
How much Dependent Choice is provable in $Z_2$? And what about Projective Determinacy?
The Feferman–Levy model does not satisfy $V=L(\Bbb R)$, whereas the model in Farmer's answer does. Truss' model, mentioned in Farmer's answer, is $L(\Bbb R)$ of the Feferman–Levy model, but it is distinct from it. In both $\omega_1$ is singular, but only in one of them (the Feferman–Levy model) the reals are a countable union of countable sets.
comment
How much Dependent Choice is provable in $Z_2$? And what about Projective Determinacy?
@Ali: The Truss model is of the form $L(\Bbb R)$, whereas the Feferman–Levy is not, in short.
comment
How much Dependent Choice is provable in $Z_2$? And what about Projective Determinacy?
@Ali: That's a misconception I had for year. But I've realised the key difference is that Truss takes all the bounded reals, and in that model the reals are not a countable union of countable sets, whereas the Feferman–Levy model has the decomposition of the reals in the model. They are very close and fairly similar, but they are different.
Loading…
comment
Breakthroughs in mathematics in 2023
But what counts as "2023"? Is it the time of announcement? arXiv preprint? Submission? Revision? Acceptance? Online publishing? Publication in print? These can all be quite different from each other.
comment
What are some nice uses of ultraproducts/ultrapowers?
@KConrad: Yes, I know several other versions of this without ultrapowers. Compactness (for countable languages, to make it choice free), absoluteness arguments to move to $L$ and do it there, which generally isn't technically an ultrapower argument in some cases, or so on. My point in this argument is to show interesting and unusual uses for logic and model theory. Doing it "without" kinda defeats the purpose...
comment
Need help in trying to understand an argument by V. A. Yankov on the nonrealizability of Scott's axiom
This works for me $\&$ ($\&$). I suspect that the reason is that you've copy-pasted it out of some PDF or whatnot, so the symbols are actually not the ASCII ones, but somewhere up there in unicode-land instead, so MathJax isn't sure what to make of the escaped symbol. If you re-type it, it seems to work.
comment
comment
Is the Ordering Principle equivalent to a selection principle?
You seem to have misread my comment, Joel. :-)
comment
Is the Ordering Principle equivalent to a selection principle?
Feels a bit convoluted, to be honest. "A set can be linearly ordered if and only if for every two points in the set we can choose a linear ordering which puts one above the other", although admittedly not as contrived.
comment
On the existence of a real which is not set-generic over $L$
Something needs to be clarified here. Just a real which is not set generic? Take any real which codes a generic for $\operatorname{Col}(\omega,V)$ and that's not going to be set generic. Why is there one? Well, why is there one which codes the universe in the Jensen case? We're working over set-models (otherwise working in $V=L$ there's no real which is not generic for the trivial forcing over $L$), so those set-models might as well be countable.
comment
The history of Proper Forcing
The link is dead. Is there an alternative source?
comment
Is the Tukey order well-founded
Consistently with ZF all ultrafilters are principal. So in that case... :)
1
7 8
9
10 11
298