Saturated models
From Peano's Parlour
The automorphism group
In a recent paper Nurkhaidarov and Schmerl 2011 proved the following: Let κ be regular, uncounale, and such that κ<κ=κ. For each completion T⊇PA let MκT be the saturated model of T of cardinality κ. There is a set T of completions of PA, such that |T|=2ℵ0 and for all T,T′∈T, if T≠T′, then \aut(MκT)≇Aut(MκT′). The following question is left open: Are there T and T′ such that T≠T′ and \aut(MκT)≅Aut(MκT′)?