example of definable type
Consider as a structure in a language
with one binary relation
, which we interpret as the order.This is a universal
, -categorical structure (see example of universal structure).
The theory of has quantifier elimination, and so is o-minimal.Thus a type over the set is determined by the quantifier free formulas over , which in turn are determined by the atomic formulas over .An atomic formula in one variable over is of the form or or for some .Thus each 1-type over determines a Dedekind cut over , and conversely a Dedekind cut determines a complete type over .Let .
Thus there are two classes of type over .
- 1.
Ones where is of the form or for some . It is clear that these are definable from the above discussion.
- 2.
Ones where has no supremum in . These are clearly not definable by o-minimality of .