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Simon Thomas
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Ultrafilters and automorphisms of the complex field

It is well-known that it is consistent with ZF that the only automorphisms of the complex field $\mathbb{C}$ are the identity map and complex conjugation. For example, we have that $|Aut(\mathbb{C}| = 2$ in $L(\mathbb{R})$. But suppose that we are given a nonprincipal ultrafilter $\mathcal{U}$ over the natural numbers $\mathbb{N}$. Is there any way to use $\mathcal{U}$ to define a third automorphism of $\mathbb{C}$?

Simon Thomas
  • 8.3k
  • 4
  • 42
  • 57