Published online by Cambridge University Press: 12 March 2014
We prove results about nonstandard formulas in models of Peano arithmetic which complement those of Kotlarski, Krajewski, and Lachlan in [KKL] and [L]. This enables us to characterize both recursive saturation and resplendency in terms of statements about nonstandard sentences. Specifically, a model of PA is recursively saturated iff
is nonstandard and
-logic is consistent.
is resplendent iff
is nonstandard,
-logic is consistent, and every sentence φ which is consistent in
-logic is contained in a full satisfaction class for
. Thus, for models of PA, recursive saturation can be expressed by a (standard)
-sentence and resplendency by a
-sentence.
The author acknowledges support of NSERC grant no. 5403.