27 research outputs found
Graph classes equivalent to 12-representable graphs
Jones et al. (2015) introduced the notion of -representable graphs, where
is a word over different from , as a generalization
of word-representable graphs. Kitaev (2016) showed that if is of length at
least 3, then every graph is -representable. This indicates that there are
only two nontrivial classes in the theory of -representable graphs:
11-representable graphs, which correspond to word-representable graphs, and
12-representable graphs. This study deals with 12-representable graphs.
Jones et al. (2015) provided a characterization of 12-representable trees in
terms of forbidden induced subgraphs. Chen and Kitaev (2022) presented a
forbidden induced subgraph characterization of a subclass of 12-representable
grid graphs.
This paper shows that a bipartite graph is 12-representable if and only if it
is an interval containment bigraph. The equivalence gives us a forbidden
induced subgraph characterization of 12-representable bipartite graphs since
the list of minimal forbidden induced subgraphs is known for interval
containment bigraphs. We then have a forbidden induced subgraph
characterization for grid graphs, which solves an open problem of Chen and
Kitaev (2022). The study also shows that a graph is 12-representable if and
only if it is the complement of a simple-triangle graph. This equivalence
indicates that a necessary condition for 12-representability presented by Jones
et al. (2015) is also sufficient. Finally, we show from these equivalences that
12-representability can be determined in time for bipartite graphs and
in time for arbitrary graphs, where and are the
number of vertices and edges of the complement of the given graph.Comment: 12 pages, 6 figure
Subclasses of Normal Helly Circular-Arc Graphs
A Helly circular-arc model M = (C,A) is a circle C together with a Helly
family \A of arcs of C. If no arc is contained in any other, then M is a proper
Helly circular-arc model, if every arc has the same length, then M is a unit
Helly circular-arc model, and if there are no two arcs covering the circle,
then M is a normal Helly circular-arc model. A Helly (resp. proper Helly, unit
Helly, normal Helly) circular-arc graph is the intersection graph of the arcs
of a Helly (resp. proper Helly, unit Helly, normal Helly) circular-arc model.
In this article we study these subclasses of Helly circular-arc graphs. We show
natural generalizations of several properties of (proper) interval graphs that
hold for some of these Helly circular-arc subclasses. Next, we describe
characterizations for the subclasses of Helly circular-arc graphs, including
forbidden induced subgraphs characterizations. These characterizations lead to
efficient algorithms for recognizing graphs within these classes. Finally, we
show how do these classes of graphs relate with straight and round digraphs.Comment: 39 pages, 13 figures. A previous version of the paper (entitled
Proper Helly Circular-Arc Graphs) appeared at WG'0
Interval-Like Graphs and Digraphs
We unify several seemingly different graph and digraph classes under one umbrella. These classes are all, broadly speaking, different generalizations of interval graphs, and include, in addition to interval graphs, adjusted interval digraphs, threshold graphs, complements of threshold tolerance graphs (known as `co-TT\u27 graphs), bipartite interval containment graphs, bipartite co-circular arc graphs, and two-directional orthogonal ray graphs. (The last three classes coincide, but have been investigated in different contexts.) This common view is made possible by introducing reflexive relationships (loops) into the analysis. We also show that all the above classes are united by a common ordering characterization, the existence of a min ordering. We propose a common generalization of all these graph and digraph classes, namely signed-interval digraphs, and show that they are precisely the digraphs that are characterized by the existence of a min ordering. We also offer an alternative geometric characterization of these digraphs. For most of the above graph and digraph classes, we show that they are exactly those signed-interval digraphs that satisfy a suitable natural restriction on the digraph, like having a loop on every vertex, or having a symmetric edge-set, or being bipartite. For instance, co-TT graphs are precisely those signed-interval digraphs that have each edge symmetric. We also offer some discussion of future work on recognition algorithms and characterizations
On some subclasses of circular-arc catch digraphs
Catch digraphs was introduced by Hiroshi Maehera in 1984 as an analog of
intersection graphs where a family of pointed sets represents a digraph. After
that Prisner continued his research particularly on interval catch digraphs by
characterizing them diasteroidal triple free. It has numerous applications in
the field of real world problems like network technology and telecommunication
operations. In this article we introduce a new class of catch digraphs, namely
circular-arc catch digraphs. The definition is same as interval catch digraph,
only the intervals are replaced by circular-arcs here. We present the
characterization of proper circular-arc catch digraphs, which is a natural
subclass of circular-arc catch digraphs where no circular-arc is contained in
other properly. We do the characterization by introducing a concept monotone
circular ordering for the vertices of the augmented adjacency matrices of it.
Next we find that underlying graph of a proper oriented circular-arc catch
digraph is a proper circular-arc graph. Also we characterize proper oriented
circular-arc catch digraphs by defining a certain kind of circular vertex
ordering of its vertices. Another interesting result is to characterize
oriented circular-arc catch digraphs which are tournaments in terms of
forbidden subdigraphs. Further we study some properties of an oriented
circular-arc catch digraph. In conclusion we discuss the relations between
these subclasses of circular-arc catch digraphs