3,945 research outputs found
Generalization of matching extensions in graphs (II)
Proposed as a general framework, Liu and Yu(Discrete Math. 231 (2001)
311-320) introduced -graphs to unify the concepts of deficiency of
matchings, -factor-criticality and -extendability. Let be a graph and
let and be non-negative integers such that and
is even. If when deleting any vertices from , the remaining
subgraph of contains a -matching and each such - matching can be
extended to a defect- matching in , then is called an
-graph. In \cite{Liu}, the recursive relations for distinct parameters
and were presented and the impact of adding or deleting an edge also
was discussed for the case . In this paper, we continue the study begun
in \cite{Liu} and obtain new recursive results for -graphs in the
general case .Comment: 12 page
Relative Expressive Power of Navigational Querying on Graphs
Motivated by both established and new applications, we study navigational
query languages for graphs (binary relations). The simplest language has only
the two operators union and composition, together with the identity relation.
We make more powerful languages by adding any of the following operators:
intersection; set difference; projection; coprojection; converse; and the
diversity relation. All these operators map binary relations to binary
relations. We compare the expressive power of all resulting languages. We do
this not only for general path queries (queries where the result may be any
binary relation) but also for boolean or yes/no queries (expressed by the
nonemptiness of an expression). For both cases, we present the complete Hasse
diagram of relative expressiveness. In particular the Hasse diagram for boolean
queries contains some nontrivial separations and a few surprising collapses.Comment: An extended abstract announcing the results of this paper was
presented at the 14th International Conference on Database Theory, Uppsala,
Sweden, March 201
Matroids with nine elements
We describe the computation of a catalogue containing all matroids with up to
nine elements, and present some fundamental data arising from this cataogue.
Our computation confirms and extends the results obtained in the 1960s by
Blackburn, Crapo and Higgs. The matroids and associated data are stored in an
online database, and we give three short examples of the use of this database.Comment: 22 page
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