# a totally categorical structure with trivial geometry which is not interpretable in the trivial structure

Among the theorems of early geometric model theory there is one by Lachlan stating that every totally categorical structure with a trivial pregeometry is intrepretable in a dense linear order.

That suggests in particular that there are totally categorical structures with a trivial pregeometry that are not interpretable in the trivial structure $(M,=)$. Could someone kindly give an example of such a structure?

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One example is the theory of a unary functions satisfying $f^2(x) =x$.
Could you tell a bit more precisely what structure you have in mind? For example, if for all $x$, $f(x)\neq x$, then this is interpetable in $(M,=)$ with the universe defined by $x \neq y$ and $f(x,y)=(y,x)$. –  Dima Sustretov Jul 16 '12 at 13:23
sorry, Neglected to say $f(x) \neq x$ and `there exist infinitely many elements' to get a complete theory. –  John Baldwin Jul 16 '12 at 13:58
Sure, but as I have said, such a structure is interpretable in $(M,=)$. Am I missing something? –  Dima Sustretov Jul 16 '12 at 14:48