M. Otero et al., ON GROUPS AND RINGS DEFINABLE IN O-MINIMAL EXPANSIONS OF REAL CLOSED FIELDS, Bulletin of the London Mathematical Society, 28, 1996, pp. 7-14
Let [R, <, +, .] be a real closed field, and let M be an o-minimal exp
ansion of R. We prove here several results regarding rings and groups
which are definable in M. We show that every M-definable ring without
zero divisors is definably isomorphic to R, R(root/(-1)) or the ring o
f quaternions over R. One corollary is that no model of T-exp is inter
pretable in a model of T-an.