102 research outputs found
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
Minimal Obstructions for Partial Representations of Interval Graphs
Interval graphs are intersection graphs of closed intervals. A generalization
of recognition called partial representation extension was introduced recently.
The input gives an interval graph with a partial representation specifying some
pre-drawn intervals. We ask whether the remaining intervals can be added to
create an extending representation. Two linear-time algorithms are known for
solving this problem.
In this paper, we characterize the minimal obstructions which make partial
representations non-extendible. This generalizes Lekkerkerker and Boland's
characterization of the minimal forbidden induced subgraphs of interval graphs.
Each minimal obstruction consists of a forbidden induced subgraph together with
at most four pre-drawn intervals. A Helly-type result follows: A partial
representation is extendible if and only if every quadruple of pre-drawn
intervals is extendible by itself. Our characterization leads to a linear-time
certifying algorithm for partial representation extension
d-representability as an embedding problem
An abstract simplicial complex is said to be -representable if it records
the intersection pattern of a collection of convex sets in . In
this paper, we show that -representability of a simplicial complex is
equivalent to the existence of a map with certain properties, from a closely
related simplicial complex into . This equivalence suggests a
framework for proving (and disproving) -representability of simplicial
complexes using topological methods such as applications of the Borsuk-Ulam
theorem, which we begin to explore.Comment: 22 pages, 7 figure
Results on Some Generalizations of Interval Graphs
An interval graph is the intersection graph of a family of intervals on the real line. Interval graphs are a well-studied class of graphs. Path graphs are a generalization of interval graphs and are defined to be the intersection graphs of a family of paths in a tree. In this thesis, we study path graphs which are representable in a subdivided K1, 3. Our main results are a characterization theorem and a polynomial time algorithm for recognition of this class of graphs. The second section of this thesis provides a bound for a graph parameter, the boxicity of a graph, for intersection graphs of subtrees of subdivided K1, n. Finally, we characterize k-trees that are path graphs
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