Abstract
It is consistent that there exists a nonprincipal ultrafilter 𝒰 over ℕ such that every automorphism of the corresponding ultraproduct is inner.
Key Words:
ACKNOWLEDGMENT
The research of the first author was supported by DFG Grant SCHI 484/4-1. The research of the second author was partially supported by NSF Grant DMS 0600940.
Notes
Communicated by D. Macpherson.