Preservation and Characterization Theorems
In this chapter we shall prove several more theorems of the general type of 24.22 and 24.23. We shall consider universal sentences, quasivarieties, universal-existential sentences, and positive sentences. We have not tried to give uniform proofs for these results, but we have instead chosen the proofs that seem most natural to us.
KeywordsRelation Symbol Characterization Theorem Operation Symbol Universal Class Elementary Extension
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