Asymptotic Theories of Classes Defined by Forbidden Homomorphisms

TL;DR

This paper characterizes the almost-sure first-order theories of classes of finite structures defined by forbidding homomorphisms, especially focusing on finite sets of oriented trees, and establishes convergence results for digraph CSPs.

Contribution

It provides a complete description of the almost-sure theories for classes defined by forbidding finite sets of oriented trees and proves first-order convergence for digraph CSPs.

Findings

All such theories are $mbda$-categorical.

Finite digraph CSPs have first-order convergence.

Asymptotic theories are finite linear combinations of $mbda$-categorical theories.

Abstract

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 sets $\mathcal{F}$ 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.