Complete theory
In
This sense of complete is distinct from the notion of a complete logic, which asserts that for every theory that can be formulated in the logic, all semantically valid statements are provable theorems (for an appropriate sense of "semantically valid"). Gödel's completeness theorem is about this latter kind of completeness.
Complete theories are closed under a number of conditions internally modelling the T-schema:
- For a set of formulas : if and only if and ,
- For a set of formulas : if and only if or .
Maximal consistent sets are a fundamental tool in the model theory of classical logic and modal logic. Their existence in a given case is usually a straightforward consequence of Zorn's lemma, based on the idea that a contradiction involves use of only finitely many premises. In the case of modal logics, the collection of maximal consistent sets extending a theory T (closed under the necessitation rule) can be given the structure of a model of T, called the canonical model.
Examples
Some examples of complete theories are:
- Presburger arithmetic
- Tarski's axioms for Euclidean geometry
- The theory of linear orderswithout endpoints
- The theory of algebraically closed fields of a given characteristic
- The theory of real closed fields
- Every uncountably categorical countabletheory
- Every countably categorical countable theory
- A group of three elements
- True arithmetic or any other elementary diagram
See also
References
- Mendelson, Elliott (1997). Introduction to Mathematical Logic (Fourth ed.). Chapman & Hall. p. 86. ISBN 978-0-412-80830-2.