5 ms·
I'm tempted to exclude the Axiom of Choice (AC) from any math I do, and instead include the Axiom of Determinacy (AD) [1] (which contradicts AC), so that all su
by tomp 7y ago
I'm tempted to exclude the Axiom of Choice (AC) from any math I do, and instead include the Axiom of Determinacy (AD) [1] (which contradicts AC), so that all subsets of R^n are measurable [2] (thus precluding the Banach–Tarski paradox), and the Axiom of Dependent Choice (DC), which is weaker than AC but sufficient to develop most of real analysis. Like, I don't really care if not all vector spaces have a basis; it's enough for me that all interesting vector spaces do (I think).
But then we have this (from [3]):
> For each of the following statements, there is some model of ZF¬C where it is true:
> - In some model, there is a set that can be partitioned into strictly more equivalence classes than the original set has elements, and a function whose domain is strictly smaller than its range. In fact, this is the case in all known models.
So it's really, pick your own poison - either one of these:
- you can take apart a ball and put the pieces back together into two balls
- there exists a function whose range is larger than its domain
Math is weird.
[1] https://en.wikipedia.org/wiki/Axiom_of_determinacy https://en.wikipedia.org/wiki/Axiom_of_determinacy
[2] https://en.wikipedia.org/wiki/Solovay_model https://en.wikipedia.org/wiki/Solovay_model
[3] https://en.wikipedia.org/wiki/Axiom_of_choice#Statements_consistent_with_the_negation_of_AC https://en.wikipedia.org/wiki/Axiom_of_choice#Statements_con...
- ginnungagap 7y agoIn fact it's an open problem (very likely the oldest open problem in set theory) whether in every model of not AC there is a set with such a paradoxical partition! Here's a nice introduction to the problem by Asaf Karagila http://karagila.org/2014/on-the-partition-principle/ http://karagila.org/2014/on-the-partition-principle/
- ot 7y ago"The Axiom of Choice is obviously true, the Well–ordering theorem is obviously false; and who can tell about Zorn’s Lemma?"