FREE GROUPS AND AUTOMORPHISM GROUPS OF INFINITE STRUCTURES

PHILIPP LÜCKE ; SAHARON SHELAH

Forum of Mathematics, Sigma, Tome 2 (2014)

Résumé

Given a cardinal $\lambda $ with $\lambda =\lambda ^{\aleph _0}$ , we show that there is a field of cardinality $\lambda $ whose automorphism group is a free group of rank $2^\lambda $ . In the proof of this statement, we develop general techniques that enable us to realize certain groups as the automorphism group of structures of a given cardinality. They allow us to show that analogues of this result hold for free objects in various varieties of groups. For example, the free abelian group of rank $2^\lambda $ is the automorphism group of a field of cardinality $\lambda $ whenever $\lambda $ is a cardinal with $\lambda =\lambda ^{\aleph _0}$ . Moreover, we apply these techniques to show that consistently the assumption that $\lambda =\lambda ^{\aleph _0}$ is not necessary for the existence of a field of cardinality $\lambda $ whose automorphism group is a free group of rank $2^\lambda $ . Finally, we use them to prove that the existence of a cardinal $\lambda $ of uncountable cofinality with the property that there is no field of cardinality $\lambda $ whose automorphism group is a free group of rank greater than $\lambda $ implies the existence of large cardinals in certain inner models of set theory.

Détail

DOI : 10.1017/fms.2014.9