WEAK HEIRS, COHEIRS, AND THE ELLIS SEMIGROUPS

Abstract Assume $G\prec H$ are groups and ${\cal A}\subseteq {\cal P}(G),\ {\cal B}\subseteq {\cal P}(H)$ are algebras of sets closed under left group translation. Under some additional assumptions we find algebraic connections between the Ellis [semi]groups of the G -flow $S({\cal A})$ and the H -flow $S({\cal B})$ . We apply these results in the model theoretic context. Namely, assume G is a group definable in a model M and $M\prec ^* N$ . Using weak heirs and weak coheirs we point out some algebraic connections between the Ellis semigroups $S_{ext,G}(M)$ and $S_{ext,G}(N)$ . Assuming every minimal left ideal in $S_{ext,G}(N)$ is a group we prove that the Ellis groups of $S_{ext,G}(M)$ are isomorphic to closed subgroups of the Ellis groups of $S_{ext,G}(N)$ .