Axiomatic Model Classes
As we saw in Chapter V, some properties of Ω-structures can be expressed by means of elementary sentences, while others cannot. We now take the analysis one step further: we consider various properties of axiomatic model classes and enquire to what extent these may be characterized by the form of the defining sentences. The chapter concludes with an elegant characterization, due to Keisler [611, of axiomatic model classes in terms of ultraproducts.
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