Minimal prime ages, words and permutation graphs

Abstract: This paper is a contribution to the study of hereditary classes of finite graphs. We classify these classes according to the number of prime structures they contain. We consider such classes that are \emph{minimal prime}: classes that contain infinitely many primes but every proper hereditary subclass contains only finitely many primes. We give a complete description of such classes. In fact, each one of these classes is a well-quasi-ordered (w.q.o) age and there are uncountably many of them. Eleven of these ages are almost multichainable; they remain w.q.o when labels in a w.q.o are added, hence have finitely many bounds. Five ages among them are exhaustible. Among the remaining ones, only countably many remain w.q.o when one label is added, and these have finitely many bounds (except for the age of the infinite path and its complement). The others have infinitely many bounds. Except for six examples, members of these ages we characterize are permutation graphs. In fact, every age which is not among the eleven ones is the age of a graph associated to a uniformly recurrent word on the integers. A description of minimal prime classes of posets and bichains is also provided. Our results support the conjecture that if a hereditary class of finite graphs does not remain w.q.o when adding labels from a w.q.o set to these graphs, then it is not w.q.o if we add just two constants to each of these graphs Our description of minimal prime classes uses a description of minimal prime graphs \cite{pouzet-zaguia2009} and previous work by Sobrani \cite{sobranithesis, sobranietat} and the authors \cite{oudrar, pouzettr} on properties of uniformly recurrent words and the associated graphs. The completeness of our description is based on classification results of Chudnovsky, Kim, Oum and Seymour \cite{chudnovsky} and Malliaris and Terry \cite {malliaris}.