Universal Structures

by Shelah. [Sh:820]

We deal with the existence of universal members in a given cardinality for several classes. First we deal with classes of Abelian groups, specifically with the existence of universal members in cardinalities which are strong limit singular of countable cofinality. Second, we then deal with (variants of) the oak property (from a work of Dzamonja and the author), a property of complete first order

theories, sufficient for the non-existence of universal models under suitable cardinal assumptions. Third, we prove that the oak property

holds for the class of groups (naturally interpreted, so for quantifier free formulas) and deal more with the existence of universals.

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