Source: Notre Dame J. Formal Logic Volume 40, Number 2
(1999), 207-226.
An open formalism for arithmetic is presented based on
first-order logic supplemented by a very strictly controlled
constructive form of the omega-rule. This formalism (which
contains Peano Arithmetic) is proved (nonconstructively, of
course) to be complete. Besides this main formalism, two other
complete open formalisms are presented, in which the only
inference rule is modus ponens. Any closure of any theorem of the
main formalism is a theorem of each of these other two. This fact
is proved constructively for the stronger of them and
nonconstructively for the weaker one. There is, though, an
interesting counterpart: the consistency of the weaker formalism
can be proved finitarily.
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