Conservative_extension Knowpia

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^{[citation needed]}: 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 edit

${\mathsf {ACA}}_{0}$ , a subsystem of second-order arithmetic studied in reverse mathematics, is a conservative extension of first-order Peano arithmetic.
The subsystems of second-order arithmetic ${\mathsf {RCA}}_{0}^{*}$ and ${\mathsf {WKL}}_{0}^{*}$ are $\Pi _{2}^{0}$ -conservative over ${\mathsf {EFA}}$ .^[1]
The subsystem ${\mathsf {WKL}}_{0}$ is a $\Pi _{1}^{1}$ -conservative extension of ${\mathsf {RCA}}_{0}$ , and a $\Pi _{2}^{0}$ -conservative over ${\mathsf {PRA}}$ (primitive recursive arithmetic).^[1]
Von Neumann–Bernays–Gödel set theory ( ${\mathsf {NBG}}$ ) is a conservative extension of Zermelo–Fraenkel set theory with the axiom of choice ( ${\mathsf {ZFC}}$ ).
Internal set theory is a conservative extension of Zermelo–Fraenkel set theory with the axiom of choice ( ${\mathsf {ZFC}}$ ).
Extensions by definitions are conservative.
Extensions by unconstrained predicate or function symbols are conservative.
$I\Sigma _{1}$ (a subsystem of Peano arithmetic with induction only for $\Sigma _{1}^{0}$ -formulas) is a $\Pi _{2}^{0}$ -conservative extension of ${\mathsf {PRA}}$ .^[2]
${\mathsf {ZFC}}$ is a $\Sigma _{3}^{1}$ -conservative extension of ${\mathsf {ZF}}$ by Shoenfield's absoluteness theorem.
${\mathsf {ZFC}}$ with the continuum hypothesis is a $\Pi _{1}^{2}$ -conservative extension of ${\mathsf {ZFC}}$ .^{[citation needed]}

Model-theoretic conservative extension edit

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.^[3] 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 edit

^ ^a ^b S. G. Simpson, R. L. Smith, "Factorization of polynomials and $\Sigma _{1}^{0}$ -induction" (1986). Annals of Pure and Applied Logic, vol. 31 (p.305)
^ 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.