FOM: reflexivity of PA
Michael Detlefsen
detlefsen.1 at nd.edu
Tue May 15 17:45:47 EDT 2001
Joe Shipman wrote:
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ZC and ZFC are not finitely axiomatizable. I know no consistent extension
of ZF is finitely axiomatizable, but is it also true that no consistent
extension of Z or ZC is finitely axiomatizable?
You can certainly replace Comprehension with "everything ZC proves about
sets of rank less than omega+omega is true", after using finitely many
instances of Replacement to define truth for V_(omega+omega), but this
won't extend ZC because you won't get ZC-theorems about sets of higher
rank. (On the other hand, you will have gotten a finitely axiomatizable
theory that will suffice for 99+% of ordinary mathematics.) If you try
instead to add an axiom of the form "there are no sets of rank omega+omega"
you won't be able to define truth for V_(omega+omega).
There are also the finitely axiomatizable subtheories ZF1, ZF2, ...where
ZFn contains the axiom "Every Pi_n theorem of ZFC is true" and the finitely
many instances of Replacement needed to define truth for Pi_n formulas. Do
these theories have any mathematical or metamathematical significance, or
any nicer finite axiomatizations?
A final question: ZF proves the consistency of any finitely axiomatizable
subtheory of itself. Do Z and PA also have this property?
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As Richard Heck indicated, the answer to the 'final question' is 'yes' for
PA. This was proved in 1952 by Mostowski in 'On Models of Axiomatic
Systems', Fundamental Mathematicae 39 (1952): 133-158, Theorem XVIII. It
was used by Mostowski (and also Montague in 1957) to conclude the
non-finite axiomatizability of PA. In the same volume of FM in which
Mostowski's paper appeared, Ryll-Nardzewski ('The Role of the Axiom of
Induction in Elementary Arithmetic') gave a more 'direct' proof of the
non-finite axiomatizability of PA.
Mic Detlefsen
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