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Asaf Karagila's user avatar
Asaf Karagila's user avatar
Asaf Karagila's user avatar
Asaf Karagila
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  • Member for 14 years, 5 months
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Indecomposable vector spaces and the axiom of choice
This example appears in Jech's The Axiom of Choice. I am well aware of this construction, my question is whether or not this sort of behaviour (where you can replace "finite subspaces" by "subspaces of cardinality $<\kappa$" for a fixed $\kappa$) can always be found in the absence of choice.
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Cohen reals and strong measure zero sets
@Andres: I know that one of the key differences between Random and Cohen reals is that Random reals are dominated by some function in the ground model, while Cohen reals are not (as you mentioned in your answer). Is this the key property for the ground model reals becoming of measure zero? (i.e. if we add one Random real, does it "nullify" (in a measure theoretic sense) the real numbers of the ground model?)
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Martin's "Philosophical Issues about the Hierarchy of Sets"
@Joel: So this philosophical argument is really like "Set theory behaves as you would expect it to behave" in terms of model isomorphisms?
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Martin's "Philosophical Issues about the Hierarchy of Sets"
Joel, as usual a very informative and interesting answer. As for your first comment, what do you mean by "full set concept"?
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Examples of common false beliefs in mathematics
@Thierry: For the past couple of weeks I spent a lot time considering models without choice, not only I held that misconception but not once anyone corrected me about it - grad students and professors alike.
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Starting PhD at the age of 25
I completely agree with Yiftach. I started my M.Sc. at 25, this last October. If I finish my Ph.D. by 30 I'll be happy about it. Maturity helps A LOT.
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(Non?)-linearity of the consistency strength ordering in ZF
I'm not even close to know enough for a proper answer, but I know that some large cardinal axioms might not be linearly ordered. You can find a graph in Kanamori's The Higher Infinite, and see there for yourself some of the possibly-non-linear sentences.
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A set for which it is hard to determine whether or not it is countable.
@David: Actually it is not very hard to add one more to a countable family, the surprising part is that it carries over to the uncountable case. For example, every countable ordinal can be embedding into the rationals, but $\omega_1$ cannot be embedded into it (even without preserving the order).
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A set for which it is hard to determine whether or not it is countable.
@Tony: I believe this discussion is only about infinite subsets of $\mathbb{N}$.
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A set for which it is hard to determine whether or not it is countable.
I believe the answer Tim is looking for is "Dedekind cuts".
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When should a supervisor be a co-author?
@Gil: Interesting, in a non-mathematical way, is the fact you treat the supervisor as a female... :-)
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Can we unify addition and multiplication into one binary operation? To what extent can we find universal binary operations?
In the introductory course I was TA'ing last semester we gave an exercise to define $\le$, $+$ and $\cdot$ from $a\mod b$ (when everything is zero when taken mod 0).
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If a result is apparently provable with AC, is actually independent of ZF?
The Cantor-Bernstein theorem was first proved using AC but is provable without it as well.
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