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Some remarks on definable equivalence relations in O-minimal structures

  • Anand Pillay (a1)

Let M be an O-minimal structure. We use our understanding, acquired in [KPS], of the structure of definable sets of n-tuples in M, to study definable (in M) equivalence relations on Mn. In particular, we show that if E is an A-definable equivalence relation on Mn (AM) then E has only finitely many classes with nonempty interior in Mn, each such class being moreover also A-definable. As a consequence, we are able to give some conditions under which an O-minimal theory T eliminates imaginaries (in the sense of Poizat [P]).

If L is a first order language and M an L-structure, then by a definable set in M, we mean something of the form XMn, n ≥ 1, where X = {(a1…,an) ∈ Mn: Mϕ(ā)} for some formula L(M). (Here L(M) means L together with names for the elements of M.) If the parameters from come from a subset A of M, we say that X is A-definable.

M is said to be O-minimal if M = (M, <,…), where < is a dense linear order with no first or last element, and every definable set XM is a finite union of points, and intervals (a, b) (where a, bM ∪ {± ∞}). (This notion is as in [PS] except here we demand the underlying order be dense.) The complete theory T is said to be O-minimal if every model of T is O-minimal. (Note that in [KPS] it is proved that if M is O-minimal, then T = Th(M) is O-minimal.) In the remainder of this section and in §2, M will denote a fixed but arbitrary O-minimal structure. A,B,C,… will denote subsets of M.

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[L2] Daniel Lascar , Sous groupes d'automorphismes d'une structure saturée. Logic Colloquium '82 ( G. Lolli et al., editors), North-Holland, Amsterdam, 1984, pp. 123134.

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The Journal of Symbolic Logic
  • ISSN: 0022-4812
  • EISSN: 1943-5886
  • URL: /core/journals/journal-of-symbolic-logic
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