Wednesday, 31 July 2013

abstract algebra - Is there an algebraic extension K/BbbQ such that textAutBbbQ(K)congBbbZ?




Is there an algebraic field extension K/Q such that AutQ(K)Z?





Here I mean the field automorphisms (which are necessarily Q-algebras automorphisms) of course.



According to this answer, one can find some extension of Q whose automorphism group is Z. But I've not seen that one can expect this extension to be algebraic.



At least such an extension can't be normal, otherwise Z would be endowed with a topology turning it into a profinite group, which can't be countably infinite.
(So typically, if we replace Q by Fp, then the answer to the above question is no, because any algebraic extension of a finite field is Galois).



Thank you!


Answer




Let L be the fixed field of AutQ(K), so Q, and K/L is a normal extension with Galois group \Bbb Z, which is impossible.


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