?0. Introduction. Let L be a first order language. If M is an L-structure, let LM be the expansion of L obtained by adding constants for the elements of M. DEFINITION. A type p(v) E Sn (M) is definable if and only if for any L-formula 0(U, T), there is an LM-formula d60(w), so that for all a c M, 0(v, a) E p u) iff M l= d6(a). The formula dO is called the definition of 0. Definable types play a central role in stability theory and have also proven useful in the study of models of arithmetic. We also remark that it is well known and easy to see that for M -< N, the property that every M-type realized in N is definable is equivalent to N being a conservative extension of M, where DEFINITION. If M -< N, we say that N is a conservative extension of M if for any n and any LN-definable S C N , S n M, is LM-definable in M. Van den Dries [DI] studied definable types over real closed fields and proved the following result. THEOREM 0.1 (van den Dries). (i) Every type over (R, ?, , 0, 1) is definable. (ii) Let F and K be real closed fields and F c K. Then, the following are equivalent: (a) Every element of K that is bounded in absolute value by an element of F is infinitely close (in the sense of F) to an element of F. (b) K is a conservative extension of F. Our goal is to provide a characterization of definable types over r-minimal structures that generalizes van den Dries' results. In particular, we will show that every type over an M-minimal expansion -of R is definable. Van des Dries has pointed out the following interesting special case of Theorem 3.4 for the &minimal expansion of R obtained by adding finitely subanalytic functions (see [D2] for definitions): (3.4.2). Let f (x, j7) be a real analytic function on I'm x In, where I is a compact interval. For a E Itm let fa(y) f (ay), and let F {fT: a E Im}. Consider rn
[1]
Lou van den Dries.
Tarski's Problem and Pfaffian Functions
,
1986
.
[2]
David Marker,et al.
Omitting Types in O-Minimal Theories
,
1986,
J. Symb. Log..
[3]
Anand Pillay.
Some Remarks on Definable Equivalence Relations in O-Minimal Structures
,
1986,
J. Symb. Log..
[4]
L. Dries.
A generalization of the Tarski-Seidenberg theorem, and some nondefinability results
,
1986
.
[5]
Anand Pillay,et al.
Discrete o-minimal structures
,
1987,
Ann. Pure Appl. Log..
[6]
A. Pillay,et al.
DEFINABLE SETS IN ORDERED STRUCTURES. I
,
1986
.