Largest initial segments pointwise fixed by automorphisms of models of set theory

Ali Enayat, Matt Kaufmann, Zachiri McKenzie

Given a model $\mathcal{M}$ of set theory, and a nontrivial automorphism $j$ of $\mathcal{M}$, let ${\mathcal{I}}_{\mathrm{fix}}(j)$ be the submodel of $\mathcal{M}$ whose universe c…