Saturday, December 9, 2017

set theory - Automorphisms on (mathbbR,+) and the Axiom of Choice



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




The argument uses the AOC.



Suppose we drop the AOC from ZFC replacing it with



Axiom (GR):



The injective mapping



Φ:R{0}AutomorphismGroup(R,+)




is surjective.






Has this 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 (R,+) to itself is continuous, and in particular of the form aar for some rR. 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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