Trees, unsplittability, property (FA) and the likes

The paper is devoted to a study of certain fixed point properties, and their relatives, in the context of full automorphism groups of countable rooted trees. Namely, we study Serre's property (FA'), also called unsplittability, property (FA), the uncountable strong cofinality, and ample generics. We give a new proof of a theorem of Psaltis to the extent that automorphism groups of rooted trees are unsplittable, and show under what circumstances automorphism groups of rooted trees have uncountable strong cofinality (and thus property (FA).) Also, we prove a necessary and sufficient condition for automorphism groups of rooted trees to have ample generics. This very strong property has interesting implications, such as the small index property, and continuity of homomorphisms. As an application, we analyze the relationship between two generalizations (discovered by Psaltis and Forester) of an interesting rigidity result on locally finite trees, originally proved by Bass and Lubotzky.

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