Asymptotic Theories of Classes Defined by Forbidden Homomorphisms

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We study the first-order almost-sure theories for classes of finite structures that are specified by homomorphically forbidding a set $\mathcal{F}$ of finite structures. If $\mathcal{F}$ consists of undirected graphs, a full description of these theories can be derived from the Kolaitis-Pr\"omel-Rothschild theorem, which treats the special case where $\mathcal{F} = \{K_n\}$. The corresponding question for finite $\mathcal{F}$ of finite directed graphs is wide open. We present a full description of the almost-sure theories of classes described by homomorphically forbidding finite sets $\mathcal{F}$ of oriented trees; all of them are $\omega$-categorical. In our proof, we establish a result of independent interest, namely that every constraint satisfaction problem for a finite digraph has first-order convergence, and that the corresponding asymptotic theory can be described as a finite linear combination of $\omega$-categorical theories.


Original languageEnglish
Publication statusPublished - 4 Apr 2022
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External IDs

ORCID /0000-0001-8228-3611/work/142659294



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