61 research outputs found
On well quasi-order of graph classes under homomorphic image orderings
In this paper we consider the question of well quasi-order for classes defined by a single obstruction within the classes of all graphs, digraphs and tournaments, under the homomorphic image ordering (in both its standard and strong forms). The homomorphic image ordering was introduced by the authors in a previous paper and corresponds to the existence of a surjective homomorphism between two structures. We obtain complete characterisations in all cases except for graphs under the strong ordering, where some open questions remain.PostprintPeer reviewe
Decidability of well quasi-order and atomicity for equivalence relations under embedding orderings
We consider the posets of equivalence relations on finite sets under the
standard embedding ordering and under the consecutive embedding ordering. In
the latter case, the relations are also assumed to have an underlying linear
order, which governs consecutive embeddings. For each poset we ask the well
quasi-order and atomicity decidability questions: Given finitely many
equivalence relations , is the downward closed set
Av consisting of all equivalence relations which do not
contain any of : (a) well-quasi-ordered, meaning that it
contains no infinite antichains? and (b) atomic, meaning that it is not a union
of two proper downward closed subsets, or, equivalently, that it satisfies the
joint embedding property
Inflations of geometric grid classes of permutations
All three authors were partially supported by EPSRC via the grant EP/J006440/1.Geometric grid classes and the substitution decomposition have both been shown to be fundamental in the understanding of the structure of permutation classes. In particular, these are the two main tools in the recent classification of permutation classes of growth rate less than κ ≈ 2.20557 (a specific algebraic integer at which infinite antichains first appear). Using language- and order-theoretic methods, we prove that the substitution closures of geometric grid classes are well partially ordered, finitely based, and that all their subclasses have algebraic generating functions. We go on to show that the inflation of a geometric grid class by a strongly rational class is well partially ordered, and that all its subclasses have rational generating functions. This latter fact allows us to conclude that every permutation class with growth rate less than κ has a rational generating function. This bound is tight as there are permutation classes with growth rate κ which have nonrational generating functions.PostprintPeer reviewe
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