Saturday, December 9, 2017

set theory - Automorphisms on $(mathbb R,+)$ and the Axiom of Choice



We know that the algebraic automorphisms of the real numbers under addition is not in $\text{1:1}$ correpondence with $\mathbb R \setminus \{0\}$; see here.




The argument uses the AOC.



Suppose we drop the AOC from $\text{ZFC}$ replacing it with



Axiom (GR):



The injective mapping



$\quad \Phi: \mathbb R \setminus \{0\} \to \text{AutomorphismGroup(} \mathbb R ,+ \text{)}$




is surjective.






Has this $\text{ZF+GR}$ been tried and/or does this lead to $1 = 0$?






Update:




Added descriptive set theory tag after looking over links in Noah's answer.


Answer



It is indeed consistent, and in fact is a consequence of the extremely powerful axiom of determinacy.



Specifically, AD implies that every homomorphism from $(\mathbb{R},+)$ to itself is continuous, and in particular of the form $a\mapsto ar$ for some $r\in\mathbb{R}$. See here for some discussion of how nasty any other endomorphism would have to be; AD rules out such sets (e.g. implies that every set of reals is measurable).



Of course, as Asaf observes below, AD is truly massive overkill (like, nuking a mosquito); I'm mentioning it because AD is a natural alternative to AC which you may independently want to know about.







Now AD isn't actually cheap: the theory ZF+AD proves the consistency of ZF, that is, the axiom determinacy is of high consistency strength. We can prove the consistency of ZF+GR relative to ZF alone; however, this is a bit more technical.


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