Published online by Cambridge University Press: 12 March 2014
Following Robinson [3], we say that a first-order sentence S is persistent with respect to a set of sentences K if, whenever S is true in a model M of K, then S is true in every extension of M that is also a model of K. Robinson proved in [3] that:
(T) In order that the sentence S be persistent with respect to the set K it is necessary and sufficient that there be some existential sentence Y such that the sentence S↔Y is deducible from K.
This note is based on part of Chapter 5 of the author's Harvard University dissertation. The author thanks Professors Michael Rabin and Burton Dreben for help in putting the results in their present form.