Uncountable Homomorphic Images of Polish Groups are not
1-Free Groups
Equipe de Logique Mathématique, Université Paris 7 2 Place Jussieu, 75251 Paris Cedex 05, France; khelif{at}logique.jussieu.fr
Received 4 November 2002. Revision received 18 March 2004.
Shelah has recently proved that an uncountable free group cannot be the automorphism group of a countable structure. In fact, he proved a more general result: an uncountable free group cannot be a Polish group. A natural question is: can an uncountable
1-free group be a Polish group? A negative answer is given here; indeed, it is proved that an
1-free group cannot be a homomorphic image of a Polish group. In fact, a stronger result is proved, involving a non-commutative analogue of the notion of separable group. 2000 Mathematics Subject Classification 20E05.