16 research outputs found
Bandwidth, expansion, treewidth, separators, and universality for bounded degree graphs
We establish relations between the bandwidth and the treewidth of bounded
degree graphs G, and relate these parameters to the size of a separator of G as
well as the size of an expanding subgraph of G. Our results imply that if one
of these parameters is sublinear in the number of vertices of G then so are all
the others. This implies for example that graphs of fixed genus have sublinear
bandwidth or, more generally, a corresponding result for graphs with any fixed
forbidden minor. As a consequence we establish a simple criterion for
universality for such classes of graphs and show for example that for each
gamma>0 every n-vertex graph with minimum degree ((3/4)+gamma)n contains a copy
of every bounded-degree planar graph on n vertices if n is sufficiently large
On the Implicit Graph Conjecture
The implicit graph conjecture states that every sufficiently small,
hereditary graph class has a labeling scheme with a polynomial-time computable
label decoder. We approach this conjecture by investigating classes of label
decoders defined in terms of complexity classes such as P and EXP. For
instance, GP denotes the class of graph classes that have a labeling scheme
with a polynomial-time computable label decoder. Until now it was not even
known whether GP is a strict subset of GR. We show that this is indeed the case
and reveal a strict hierarchy akin to classical complexity. We also show that
classes such as GP can be characterized in terms of graph parameters. This
could mean that certain algorithmic problems are feasible on every graph class
in GP. Lastly, we define a more restrictive class of label decoders using
first-order logic that already contains many natural graph classes such as
forests and interval graphs. We give an alternative characterization of this
class in terms of directed acyclic graphs. By showing that some small,
hereditary graph class cannot be expressed with such label decoders a weaker
form of the implicit graph conjecture could be disproven.Comment: 13 pages, MFCS 201
Universality for graphs of bounded degeneracy
Given a family of graphs, a graph is called
-universal if contains every graph of as a
subgraph. Following the extensive research on universal graphs of small size
for bounded-degree graphs, Alon asked what is the minimum number of edges that
a graph must have to be universal for the class of all -vertex graphs that
are -degenerate. In this paper, we answer this question up to a factor that
is polylogarithmic in Comment: 17 page
Universality for graphs with bounded density
A graph is for a (finite) family of
graphs if every is a subgraph of . For a given family
, the goal is to determine the smallest number of edges an
-universal graph can have. With the aim of unifying a number of
recent results, we consider a family of graphs with bounded density. In
particular, we construct a graph with edges which contains every -vertex graph with density at most
(), which is close to a lower bound obtained by counting lifts of a carefully chosen (small) graph.
When restricting the maximum degree of such graphs to be constant, we obtain a
near-optimal universality. If we further assume , we get an
asymptotically optimal construction.Comment: 14 pages, updated version focusing on density, with new title and
additional autho
Sparse universal graphs
AbstractFor every n, we describe an explicit construction of a graph on n vertices with at most O(n2−ε) edges, for ε=0.133…, that contains every graph on n vertices with maximum degree 3 as a subgraph. It is easy to see that each such graph must have at least Ω(n4/3) edges. We also show that the minimum number of edges of a graph that contains every graph with n edges as a subgraph is Θ(n2/(log2n)). This improves a result of Babai, Chung, Erdös, Graham and Spencer (Ann. Discrete Math. 12 (1982) 21–26)
Near-optimum universal graphs for graphs with bounded degrees (Extended abstract)
Let H be a family of graphs. We say that G is H-universal if, for each H ∈H, the graph G contains a subgraph isomorphic to H. Let H(k, n) denote the family of graphs on n vertices with maximum degree at most k. For each fixed k and each n sufficiently large, we explicitly construct an H(k, n)-universal graph Γ(k, n) with O(n2−2/k(log n)1+8/k) edges. This is optimal up to a small polylogarithmic factor, as Ω(n2−2/k) is a lower bound for the number of edges in any such graph. En route, we use the probabilistic method in a rather unusual way. After presenting a deterministic construction of the graph Γ(k, n), we prove, using a probabilistic argument, that Γ(k, n) is H(k, n)-universal. So we use the probabilistic method to prove that an explicit construction satisfies certain properties, rather than showing the existence of a construction that satisfies these properties. © Springer-Verlag Berlin Heidelberg 200
Size Ramsey Number of Bounded Degree Graphs for Games
We study Maker/Breaker games on the edges of sparse graphs. Maker and Breaker take turns at claiming previously unclaimed edges of a given graph H. Maker aims to occupy a given target graph G and Breaker tries to prevent Maker from achieving his goal. We show that for every d there is a constant c = c(d) with the property that for every graph G on n vertices of maximum degree d there is a graph H on at most cn edges such that Maker has a strategy to occupy a copy of G in the game on H. This is a result about a game-theoretic variant of the size Ramsey number. For a given graph G, is defined as the smallest number M for which there exists a graph H with M edges such that Maker has a strategy to occupy a copy of G in the game on H. In this language, our result yields that for every connected graph G of constant maximum degree, . Moreover, we can also use our method to settle the corresponding extremal number for universal graphs: for a constant d and for the class of n-vertex graphs of maximum degree d, denotes the minimum number such that there exists a graph H with M edges where, for every G ∈ , Maker has a strategy to build a copy of G in the game on H. We obtain that $s({\cal G}_{n}) = \Theta(n^{2 - \frac{2}{d}})
Universal Geometric Graphs
We introduce and study the problem of constructing geometric graphs that have
few vertices and edges and that are universal for planar graphs or for some
sub-class of planar graphs; a geometric graph is \emph{universal} for a class
of planar graphs if it contains an embedding, i.e., a
crossing-free drawing, of every graph in .
Our main result is that there exists a geometric graph with vertices and
edges that is universal for -vertex forests; this extends to
the geometric setting a well-known graph-theoretic result by Chung and Graham,
which states that there exists an -vertex graph with edges
that contains every -vertex forest as a subgraph. Our bound on
the number of edges cannot be improved, even if more than vertices are
allowed.
We also prove that, for every positive integer , every -vertex convex
geometric graph that is universal for -vertex outerplanar graphs has a
near-quadratic number of edges, namely ; this almost
matches the trivial upper bound given by the -vertex complete
convex geometric graph.
Finally, we prove that there exists an -vertex convex geometric graph with
vertices and edges that is universal for -vertex
caterpillars.Comment: 20 pages, 8 figures; a 12-page extended abstracts of this paper will
appear in the Proceedings of the 46th Workshop on Graph-Theoretic Concepts in
Computer Science (WG 2020