# Automorphism groups in general

From Peano's Parlour

## Rigid models

A model $M$ is **rigid** if ${\rm Aut}(M)$ is trivial -- that is, its only automorphism
is the identity function. Every model of PA has a rigid elementary end extension.
(See [1] for a proof of this and much more.)

Question: Does every nonstandard model of PA have a rigid cofinal extension?