82,434 research outputs found
On grounded L-graphs and their relatives
We consider the graph class Grounded-L corresponding to graphs that admit an
intersection representation by L-shaped curves, where additionally the topmost
points of each curve are assumed to belong to a common horizontal line. We
prove that Grounded-L graphs admit an equivalent characterisation in terms of
vertex ordering with forbidden patterns.
We also compare this class to related intersection classes, such as the
grounded segment graphs, the monotone L-graphs (a.k.a. max point-tolerance
graphs), or the outer-1-string graphs. We give constructions showing that these
classes are all distinct and satisfy only trivial or previously known
inclusions.Comment: 16 pages, 6 figure
On the intersection of tolerance and cocomparability graphs.
It has been conjectured by Golumbic and Monma in 1984 that the intersection of tolerance and cocomparability graphs coincides with bounded tolerance graphs. Since cocomparability graphs can be efficiently recognized, a positive answer to this conjecture in the general case would enable us to efficiently distinguish between tolerance and bounded tolerance graphs, although it is NP-complete to recognize each of these classes of graphs separately. The conjecture has been proved under some – rather strong – structural assumptions on the input graph; in particular, it has been proved for complements of trees, and later extended to complements of bipartite graphs, and these are the only known results so far. Furthermore, it is known that the intersection of tolerance and cocomparability graphs is contained in the class of trapezoid graphs. In this article we prove that the above conjecture is true for every graph G, whose tolerance representation satisfies a slight assumption; note here that this assumption concerns only the given tolerance representation R of G, rather than any structural property of G. This assumption on the representation is guaranteed by a wide variety of graph classes; for example, our results immediately imply the correctness of the conjecture for complements of triangle-free graphs (which also implies the above-mentioned correctness for complements of bipartite graphs). Our proofs are algorithmic, in the sense that, given a tolerance representation R of a graph G, we describe an algorithm to transform R into a bounded tolerance representation R  ∗  of G. Furthermore, we conjecture that any minimal tolerance graph G that is not a bounded tolerance graph, has a tolerance representation with exactly one unbounded vertex. Our results imply the non-trivial result that, in order to prove the conjecture of Golumbic and Monma, it suffices to prove our conjecture. In addition, there already exists evidence in the literature that our conjecture is true
Constellation Queries over Big Data
A geometrical pattern is a set of points with all pairwise distances (or,
more generally, relative distances) specified. Finding matches to such patterns
has applications to spatial data in seismic, astronomical, and transportation
contexts. For example, a particularly interesting geometric pattern in
astronomy is the Einstein cross, which is an astronomical phenomenon in which a
single quasar is observed as four distinct sky objects (due to gravitational
lensing) when captured by earth telescopes. Finding such crosses, as well as
other geometric patterns, is a challenging problem as the potential number of
sets of elements that compose shapes is exponentially large in the size of the
dataset and the pattern. In this paper, we denote geometric patterns as
constellation queries and propose algorithms to find them in large data
applications. Our methods combine quadtrees, matrix multiplication, and
unindexed join processing to discover sets of points that match a geometric
pattern within some additive factor on the pairwise distances. Our distributed
experiments show that the choice of composition algorithm (matrix
multiplication or nested loops) depends on the freedom introduced in the query
geometry through the distance additive factor. Three clearly identified blocks
of threshold values guide the choice of the best composition algorithm.
Finally, solving the problem for relative distances requires a novel
continuous-to-discrete transformation. To the best of our knowledge this paper
is the first to investigate constellation queries at scale
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