139,719 research outputs found
The VC-Dimension of Graphs with Respect to k-Connected Subgraphs
We study the VC-dimension of the set system on the vertex set of some graph
which is induced by the family of its -connected subgraphs. In particular,
we give tight upper and lower bounds for the VC-dimension. Moreover, we show
that computing the VC-dimension is -complete and that it remains
-complete for split graphs and for some subclasses of planar
bipartite graphs in the cases and . On the positive side, we
observe it can be decided in linear time for graphs of bounded clique-width
Automorphism Groups of Geometrically Represented Graphs
We describe a technique to determine the automorphism group of a
geometrically represented graph, by understanding the structure of the induced
action on all geometric representations. Using this, we characterize
automorphism groups of interval, permutation and circle graphs. We combine
techniques from group theory (products, homomorphisms, actions) with data
structures from computer science (PQ-trees, split trees, modular trees) that
encode all geometric representations.
We prove that interval graphs have the same automorphism groups as trees, and
for a given interval graph, we construct a tree with the same automorphism
group which answers a question of Hanlon [Trans. Amer. Math. Soc 272(2), 1982].
For permutation and circle graphs, we give an inductive characterization by
semidirect and wreath products. We also prove that every abstract group can be
realized by the automorphism group of a comparability graph/poset of the
dimension at most four
Homology surgery and invariants of 3-manifolds
We introduce a homology surgery problem in dimension 3 which has the property
that the vanishing of its algebraic obstruction leads to a canonical class of
\pi-algebraically-split links in 3-manifolds with fundamental group \pi . Using
this class of links, we define a theory of finite type invariants of
3-manifolds in such a way that invariants of degree 0 are precisely those of
conventional algebraic topology and surgery theory. When finite type invariants
are reformulated in terms of clovers, we deduce upper bounds for the number of
invariants in terms of \pi-decorated trivalent graphs. We also consider an
associated notion of surgery equivalence of \pi-algebraically split links and
prove a classification theorem using a generalization of Milnor's
\mu-invariants to this class of links.Comment: Published in Geometry and Topology at
http://www.maths.warwick.ac.uk/gt/GTVol5/paper18.abs.htm
Clique versus Independent Set
Yannakakis' Clique versus Independent Set problem (CL-IS) in communication
complexity asks for the minimum number of cuts separating cliques from stable
sets in a graph, called CS-separator. Yannakakis provides a quasi-polynomial
CS-separator, i.e. of size , and addresses the problem of
finding a polynomial CS-separator. This question is still open even for perfect
graphs. We show that a polynomial CS-separator almost surely exists for random
graphs. Besides, if H is a split graph (i.e. has a vertex-partition into a
clique and a stable set) then there exists a constant for which we find a
CS-separator on the class of H-free graphs. This generalizes a
result of Yannakakis on comparability graphs. We also provide a
CS-separator on the class of graphs without induced path of length k and its
complement. Observe that on one side, is of order
resulting from Vapnik-Chervonenkis dimension, and on the other side, is
exponential.
One of the main reason why Yannakakis' CL-IS problem is fascinating is that
it admits equivalent formulations. Our main result in this respect is to show
that a polynomial CS-separator is equivalent to the polynomial
Alon-Saks-Seymour Conjecture, asserting that if a graph has an edge-partition
into k complete bipartite graphs, then its chromatic number is polynomially
bounded in terms of k. We also show that the classical approach to the stubborn
problem (arising in CSP) which consists in covering the set of all solutions by
instances of 2-SAT is again equivalent to the existence of a
polynomial CS-separator
Locating-dominating sets in twin-free graphs
A locating-dominating set of a graph is a dominating set of with
the additional property that every two distinct vertices outside have
distinct neighbors in ; that is, for distinct vertices and outside
, where denotes the open neighborhood
of . A graph is twin-free if every two distinct vertices have distinct open
and closed neighborhoods. The location-domination number of , denoted
, is the minimum cardinality of a locating-dominating set in .
It is conjectured [D. Garijo, A. Gonz\'alez and A. M\'arquez. The difference
between the metric dimension and the determining number of a graph. Applied
Mathematics and Computation 249 (2014), 487--501] that if is a twin-free
graph of order without isolated vertices, then . We prove the general bound ,
slightly improving over the bound of Garijo et
al. We then provide constructions of graphs reaching the bound,
showing that if the conjecture is true, the family of extremal graphs is a very
rich one. Moreover, we characterize the trees that are extremal for this
bound. We finally prove the conjecture for split graphs and co-bipartite
graphs.Comment: 11 pages; 4 figure
The local metric dimension of split and unicyclic graphs
A set W is called a local resolving set of G if the distance of u and v to some elements of W are distinct for every two adjacent vertices u and v in G. The local metric dimension of G is the minimum cardinality of a local resolving set of G. A connected graph G is called a split graph if V(G) can be partitioned into two subsets V1 and V2 where an induced subgraph of G by V1 and V2 is a complete graph and an independent set, respectively. We also consider a graph, namely the unicyclic graph which is a connected graph containing exactly one cycle. In this paper, we provide a general sharp bounds of local metric dimension of split graph. We also determine an exact value of local metric dimension of any unicyclic graphs
The (weighted) metric dimension of graphs : hard and easy cases
Given an input undirected graph G=(V,E), we say that a vertex l separates u from v (where u,v ¿ V) if the distance between u and l differs from the distance from v to l. A set of vertices L¿V is a feasible solution if for every pair of vertices, u,v ¿ V (u¿v), there is a vertex l ¿ L that separates u from v. Such a feasible solution is called a landmark set, and the metric dimension of a graph is the minimum cardinality of a landmark set. Here, we extend this well-studied problem to the case where each vertex v has a non-negative cost, and the goal is to find a feasible solution with a minimum total cost. This weighted version is NP-hard since the unweighted variant is known to be NP-hard. We show polynomial time algorithms for the cases where G is a path, a tree, a cycle, a cograph, a k-edge-augmented tree (that is, a tree with additional k edges) for a constant value of k, and a (not necessarily complete) wheel. The results for paths, trees, cycles, and complete wheels extend known polynomial time algorithms for the unweighted version, whereas the other results are the first known polynomial time algorithms for these classes of graphs even for the unweighted version. Next, we extend the set of graph classes for which computing the unweighted metric dimension of a graph is known to be NP-hard. We show that for split graphs, bipartite graphs, co-bipartite graphs, and line graphs of bipartite graphs, the problem of computing the unweighted metric dimension of the graph is NP-hard
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