Summary
LetT be a universal theory of graphs such that Mod(T) is closed under disjoint unions. Letℳ T be a disjoint union
ℳ i such that eachℳ i is a finite model ofT and every finite isomorphism type in Mod(T) is represented in{ℳ i ∶i<Ω3}. We investigate under what conditions onT, Th(ℳ T ) is a coinductive theory, where a theory is called coinductive if it can be axiomatizated by ∃∀-sentences. We also characterize coinductive graphs which have quantifier-free rank 1.
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Lachlan, A.H. Some coinductive graphs. Arch Math Logic 29, 213–229 (1990). https://doi.org/10.1007/BF01651325
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DOI: https://doi.org/10.1007/BF01651325