# Elementary extensions and type spaces

If $M$ and $N$ are two $L$-structures, and $f: M \rightarrow N$ is an elementary extension, then given any subset $A$ of $M$, $f$ induces in a natural way a morphism $S^M_n(A) \rightarrow S^N_n(f(A))$ of type spaces which is in fact a homeomorphism of Stone spaces (in fact the underlying sets are equal: see the answer below).

What results exist about converse statements? If two type spaces are equal, when can you say it's because of the existence of an elementary embedding? What if you have a non-trivial homeomorphism, or some more general injective continuous map? Can you sometimes conclude that that it's induced by a suitable morphism between the structures?

The two type spaces that you are considering are equal: types over $A$ are the same thing whether we look at $A$ in $M$ or in $N$.
An element of $S_n^M(A)$ is a maximal consistent set of formulas with parameters in $A$ and free variables $x_1,...,x_n$ containing $T_M(A)$, where $T_M(A)$ is the set of sentences with parameters in $A$ that are true in $M$. By definition of an elementary extension, $T_N(f(A))=T_M(A)$: the same sentences are satisfied by $A$ and by $f(A)$. Hence also $S_n^M(A)=S_n^N(f(A))$.