14 research outputs found
Caterpillar dualities and regular languages
We characterize obstruction sets in caterpillar dualities in terms of regular
languages, and give a construction of the dual of a regular family of
caterpillars. We show that these duals correspond to the constraint
satisfaction problems definable by a monadic linear Datalog program with at
most one EDB per rule
On infinite-finite duality pairs of directed graphs
The (A,D) duality pairs play crucial role in the theory of general relational
structures and in the Constraint Satisfaction Problem. The case where both
classes are finite is fully characterized. The case when both side are infinite
seems to be very complex. It is also known that no finite-infinite duality pair
is possible if we make the additional restriction that both classes are
antichains. In this paper (which is the first one of a series) we start the
detailed study of the infinite-finite case.
Here we concentrate on directed graphs. We prove some elementary properties
of the infinite-finite duality pairs, including lower and upper bounds on the
size of D, and show that the elements of A must be equivalent to forests if A
is an antichain. Then we construct instructive examples, where the elements of
A are paths or trees. Note that the existence of infinite-finite antichain
dualities was not previously known
Generalised dualities and maximal finite antichains in the homomorphism order of relational structures
The motivation for this paper is threefold. First, we study the connectivity properties of the homomorphism order of directed graphs, and more generally for relational structures. As opposed to the homomorphism order of undirected graphs (which has no non-trivial finite maximal antichains), the order of directed graphs has finite maximal antichains of any size. In this paper, we characterise explicitly all maximal antichains in the homomorphism order of directed graphs.
Quite surprisingly, these maximal antichains correspond to generalised dualities. The notion of generalised duality is defined here in full generality as an extension of the notion of finitary duality, investigated in [J. Nešetřil, C. Tardif, Duality theorems for finite structures (characterising gaps and good characterisations), J. Combin. Theory Ser. B 80 (1) (2000) 80–97]. Building upon the results of the cited paper, we fully characterise the generalised dualities. It appears that these dualities are determined by forbidding homomorphisms from a finite set of forests (rather than trees).
Finally, in the spirit of [A. Atserias, On digraph coloring problems and treewidth duality, in: Proceedings of the 21st IEEE Symposium on Logic in Computer Science, LICS’06, IEEE Computer Society, 2006; B. Larose, C. Loten, C. Tardif, A characterisation of first-order constraint satisfaction problems, in: Proceedings of the 21st IEEE Symposium on Logic in Computer Science, LICS’06, IEEE Computer Society, 2006; V. Dalmau, A. Krokhin, B. Larose, First-order definable retraction problems for posets and reflexive graphs, in: Proceedings of the 19th IEEE Symposium on Logic in Computer Science, LICS’04, IEEE Computer Society, 2004 [5]] we shall characterise “generalised” constraint satisfaction problems (defined also here) that are first-order definable. These are again just generalised dualities corresponding to finite maximal antichains in the homomorphism order
Adjoint functors and tree duality
A family T of digraphs is a complete set of obstructions for a digraph H if
for an arbitrary digraph G the existence of a homomorphism from G to H is
equivalent to the non-existence of a homomorphism from any member of T to G. A
digraph H is said to have tree duality if there exists a complete set of
obstructions T consisting of orientations of trees. We show that if H has tree
duality, then its arc graph delta H also has tree duality, and we derive a
family of tree obstructions for delta H from the obstructions for H.
Furthermore we generalise our result to right adjoint functors on categories
of relational structures. We show that these functors always preserve tree
duality, as well as polynomial CSPs and the existence of near-unanimity
functions.Comment: 14 pages, 2 figures; v2: minor revision
Rewritability in Monadic Disjunctive Datalog, MMSNP, and Expressive Description Logics
We study rewritability of monadic disjunctive Datalog programs, (the
complements of) MMSNP sentences, and ontology-mediated queries (OMQs) based on
expressive description logics of the ALC family and on conjunctive queries. We
show that rewritability into FO and into monadic Datalog (MDLog) are decidable,
and that rewritability into Datalog is decidable when the original query
satisfies a certain condition related to equality. We establish
2NExpTime-completeness for all studied problems except rewritability into MDLog
for which there remains a gap between 2NExpTime and 3ExpTime. We also analyze
the shape of rewritings, which in the MMSNP case correspond to obstructions,
and give a new construction of canonical Datalog programs that is more
elementary than existing ones and also applies to formulas with free variables
Universal intervals in the homomorphism order of digraphs
In this thesis we solve some open problems related to the homomorphism order of digraphs. We begin by introducing the basic concepts of graphs and homomorphisms and studying some properties of the homomorphism order of digraphs. Then we present the new results. First, we show that the class of digraphs containing cycles has the fractal property (strengthening the density property) . Then we show a density theorem for the class of proper oriented trees. Here we say that a tree is proper if it is not a path. Such result was claimed in 2005 but none proof have been published ever since. We also show that the class of proper oriented trees, in addition to be dense, has the fractal property. We end by considering the consequences of these results and the remaining open questions in this area.Outgoin