Conservative extension

In mathematical logic, a conservative extension is a supertheory of a theory which is often convenient for proving theorems, but proves no new theorems about the language of the original theory. Similarly, a non-conservative extension is a supertheory which is not conservative, and can prove more theorems than the original.

More formally stated, a theory $T_{2}$ is a (proof theoretic) conservative extension of a theory $T_{1}$ if every theorem of $T_{1}$ is a theorem of $T_{2}$ , and any theorem of $T_{2}$ in the language of $T_{1}$ is already a theorem of $T_{1}$ .

More generally, if $\Gamma$ is a set of formulas in the common language of $T_{1}$ and $T_{2}$ , then $T_{2}$ is $\Gamma$ -conservative over $T_{1}$ if every formula from $\Gamma$ provable in $T_{2}$ is also provable in $T_{1}$ .

Note that a conservative extension of a consistent theory is consistent. If it were not, then by the principle of explosion, every formula in the language of $T_{2}$ would be a theorem of $T_{2}$ , so every formula in the language of $T_{1}$ would be a theorem of $T_{1}$ , so $T_{1}$ would not be consistent. Hence, conservative extensions do not bear the risk of introducing new inconsistencies. This can also be seen as a methodology for writing and structuring large theories: start with a theory, $T_{0}$ , that is known (or assumed) to be consistent, and successively build conservative extensions $T_{1}$ , $T_{2}$ , ... of it.

Recently, conservative extensions have been used for defining a notion of module for ontologies: if an ontology is formalized as a logical theory, a subtheory is a module if the whole ontology is a conservative extension of the subtheory.

An extension which is not conservative may be called a proper extension.

Examples

ACA₀ (a subsystem of second-order arithmetic) is a conservative extension of first-order Peano arithmetic.
Von Neumann–Bernays–Gödel set theory is a conservative extension of Zermelo–Fraenkel set theory with the axiom of choice (ZFC).
Internal set theory is a conservative extension of Zermelo–Fraenkel set theory with the axiom of choice (ZFC).
Extensions by definitions are conservative.
Extensions by unconstrained predicate or function symbols are conservative.
IΣ₁ (a subsystem of Peano arithmetic with induction only for Σ⁰₁-formulas) is a Π⁰₂-conservative extension of the primitive recursive arithmetic (PRA).^[1]
ZFC is a Π¹₃-conservative extension of ZF by Shoenfield's absoluteness theorem.
ZFC with the continuum hypothesis is a Π²₁-conservative extension of ZFC.

Model-theoretic conservative extension

With model-theoretic means, a stronger notion is obtained: an extension $T_{2}$ of a theory $T_{1}$ is model-theoretically conservative if $T_{1}\subseteq T_{2}$ and every model of $T_{1}$ can be expanded to a model of $T_{2}$ . Each model-theoretic conservative extension also is a (proof-theoretic) conservative extension in the above sense.^[2] The model theoretic notion has the advantage over the proof theoretic one that it does not depend so much on the language at hand; on the other hand, it is usually harder to establish model theoretic conservativity.

References

^ Fernando Ferreira, A Simple Proof of Parsons’ Theorem. Notre Dame Journal of Formal Logic, Vol.46, No.1, 2005.
^ Hodges, Wilfrid (1997). A shorter model theory. Cambridge: Cambridge University Press. p. 58 exercise 8. ISBN 978-0-521-58713-6.

External links

The importance of conservative extensions for the foundations of mathematics

[1] Fernando Ferreira, A Simple Proof of Parsons’ Theorem. Notre Dame Journal of Formal Logic, Vol.46, No.1, 2005.

[2] Hodges, Wilfrid (1997). A shorter model theory. Cambridge: Cambridge University Press. p. 58 exercise 8. ISBN 978-0-521-58713-6.

[1]

[2]

Conservative extension

Examples

Model-theoretic conservative extension

References

See also

External links