Hereditary classes of ordered binary structures

Abstract: Balogh, Bollob\'{a}s and Morris (2006) have described a threshold phenomenon in the behavior of the profile of hereditary classes of ordered graphs. In this paper, we give an other look at their result based on the notion of monomorphic decomposition of a relational structure introduced in \cite{P-T-2013}. We prove that the class $\mathfrak S$ of ordered binary structures which do not have a finite monomorphic decomposition has a finite basis (a subset $\mathfrak A$ such that every member of $\mathfrak S$ embeds some member of $\mathfrak A$). In the case of ordered reflexive directed graphs, the basis has 1242 members and the profile of their ages grows at least as the Fibonacci function. From this result, we deduce that the following dichotomy property holds for every hereditary class $\mathfrak C$ of finite ordered binary structures of a given finite type. Either there is an integer $\ell$ such that every member of $\mathfrak C$ has a monomorphic decomposition into at most $\ell$ blocks and in this case the profile of $\mathfrak C$ is bounded by a polynomial of degree $\ell-1$ (and in fact is a polynomial), or $\mathfrak C$ contains the age of a structure which does not have a finite monomorphic decomposition, in which case the profile of $\mathfrak C$ is bounded below by the Fibonacci function.