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Omitting types for finite variable fragments and complete representations of algebras. (English) Zbl 1143.03035
A novel application of algebraic logic to first-order logic is given. The last author has already proved in [Stud. Log. 72, No. 2, 285–309 (2002; Zbl 1019.03047)] that a version of the omitting types theorem fails, in a certain sense, in finite variable fragments of first-order logic; also, complete representations were discussed there. In the paper under review, a new construction is presented for representable but not completely representable atomic relation and cylindric algebras of dimension $$n$$ with $$n > 2$$. In particular, the constructed algebras are one-generated. This construction is used to prove a stronger version of the mentioned negative result, which goes back to a conjecture in [loc. cit.]. It is shown that one binary relation in the language suffices, and that there is no upper bound on the number of extra variables needed in the so-called witness formula. The proof of the new theorem uses methods of algebraic logic; the theorem is actually given also an algebraic form that is even stronger than the logical one.

##### MSC:
 03G15 Cylindric and polyadic algebras; relation algebras 03B10 Classical first-order logic 03C40 Interpolation, preservation, definability
Zbl 1019.03047
Full Text:
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