Model complete theory explained

In model theory, a first-order theory is called model complete if every embedding of its models is an elementary embedding. Equivalently, every first-order formula is equivalent to a universal formula.This notion was introduced by Abraham Robinson.

Model companion and model completion

A companion of a theory T is a theory T* such that every model of T can be embedded in a model of T* and vice versa.

A model companion of a theory T is a companion of T that is model complete. Robinson proved that a theory has at most one model companion. Not every theory is model-companionable, e.g. theory of groups. However if T is an

\aleph _{0} -categorical theory, then it always has a model companion.

A model completion for a theory T is a model companion T* such that for any model M of T, the theory of T* together with the diagram of M is complete. Roughly speaking, this means every model of T is embeddable in a model of T* in a unique way.

If T* is a model companion of T then the following conditions are equivalent:

• T* is a model completion of T
• T has the amalgamation property.

If T also has universal axiomatization, both of the above are also equivalent to:

• T* has elimination of quantifiers

Examples

• Any theory with elimination of quantifiers is model complete.
• The theory of algebraically closed fields is the model completion of the theory of fields. It is model complete but not complete.
• The model completion of the theory of equivalence relations is the theory of equivalence relations with infinitely many equivalence classes, each containing an infinite number of elements.
• The theory of real closed fields, in the language of ordered rings, is a model completion of the theory of ordered fields (or even ordered domains).
• The theory of real closed fields, in the language of rings, is the model companion for the theory of formally real fields, but is not a model completion.

Non-examples

• The theory of dense linear orders with a first and last element is complete but not model complete.
• The theory of groups (in a language with symbols for the identity, product, and inverses) has the amalgamation property but does not have a model companion.

Sufficient condition for completeness of model-complete theories

If T is a model complete theory and there is a model of T that embeds into any model of T, then T is complete.

References

• Book: Chang. Chen Chung. Chen Chung Chang. Keisler. H. Jerome. Howard Jerome Keisler. Model Theory. 1990. 1973. Elsevier. 3rd. Studies in Logic and the Foundations of Mathematics. 978-0-444-88054-3.
• Book: Chang. Chen Chung. Chen Chung Chang. Keisler. H. Jerome. Howard Jerome Keisler. Model Theory. 2012. 1990. Dover Publications. 3rd. Dover Books on Mathematics. 672. 978-0-486-48821-9.
• Book: Hirschfeld, Joram. Wheeler. William H.. Model-completions and model-companions. Forcing, Arithmetic, Division Rings. Lecture Notes in Mathematics. Springer. 454. 44–54. 1975. 978-3-540-07157-0. 0389581. 10.1007/BFb0064085.
• Book: Marker, David . Model Theory: An Introduction . Springer-Verlag. New York. 2002 . 0-387-98760-6. Graduate Texts in Mathematics 217.
• Saracino. D.. Model Companions for ℵ₀-Categorical Theories. Proceedings of the American Mathematical Society. 39. 3. August 1973. 591–598.
• Simmons. H.. Large and Small Existentially Closed Structures. Journal of Symbolic Logic. 41. 2. 1976. 379–390.