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Abstract: A complete theory T is called geometric if the algebraic closure has the exchange property in all models of T and thetheory eliminates the quantifier exists infinity. In such theories there is a rudimentary notion of independence given by algebraic independence. Examples of geometric theories include SU-rank one theories and dense o-minimal theories.

An expansion of a model M of T by a unary predicate H is called dense-codense if for every finite dimensional subset A of M and every non algebraic type p(x) over A, there is a realization of p(x) in H(M) and another one which is not algebraic over AH(M).A dense-codense expansion is called an H-structure if in addition H(M) is algebraically independent.This talk includes joint work with E. Vassiliev, D. Garcia and T. Zou.