14 research outputs found
Edge-superconnectivity of semiregular cages with odd girth
A graph is said to be edge-superconnected if each minimum edge-cut consists of all the edges incident with some vertex of minimum degree. A graph G is said to be a {d, d + 1}-
semiregular graph if all its vertices have degree either d or d+1. A smallest {d,d+1}-semiregular graph G with girth g is said to be a ({d, d+1}; g)-cage.We show that every ({d, d+1}; g)-cage with odd girth g is edge-superconnected.Peer Reviewe
On the structure of graphs without short cycles
The objective of this thesis is to study cages, constructions and properties of such families of graphs. For this, the study of graphs without short cycles plays a fundamental role in order to develop some knowledge on their structure, so we can later deal with the problems on cages. Cages were introduced by Tutte in 1947. In 1963, Erdös and Sachs proved that (k, g) -cages exist for any given values of k and g. Since then, large amount of research in cages has been devoted to their construction.
In this work we study structural properties such as the connectivity, diameter, and degree regularity of graphs without short cycles.
In some sense, connectivity is a measure of the reliability of a network. Two graphs with the same edge-connectivity, may be considered to have different reliabilities, as a more refined index than the edge-connectivity, edge-superconnectivity is proposed together with some other parameters called restricted connectivities.
By relaxing the conditions that are imposed for the graphs to be cages, we can achieve more refined connectivity properties on these families and also we have an approach to structural properties of the family of graphs with more restrictions (i.e., the cages).
Our aim, by studying such structural properties of cages is to get a deeper insight into their structure so we can attack the problem of their construction.
By way of example, we studied a condition on the diameter in relation to the girth pair of a graph, and as a corollary we obtained a result guaranteeing restricted connectivity of a special family of graphs arising from geometry, such as polarity graphs.
Also, we obtained a result proving the edge superconnectivity of semiregular cages. Based on these studies it was possible to develop the study of cages.
Therefore obtaining a relevant result with respect to the connectivity of cages, that is, cages are k/2-connected. And also arising from the previous work on girth pairs we obtained constructions for girth pair cages that proves a bound conjectured by Harary and Kovács, relating the order of girth pair cages with the one for cages. Concerning the degree and the diameter, there is the concept of a Moore graph, it was introduced by Hoffman and Singleton after Edward F. Moore, who posed the question of describing and classifying these graphs.
As well as having the maximum possible number of vertices for a given combination of degree and diameter, Moore graphs have the minimum possible number of vertices for a regular graph with given degree and girth. That is, any Moore graph is a cage. The formula for the number of vertices in a Moore graph can be generalized to allow a definition of Moore graphs with even girth (bipartite Moore graphs) as well as odd girth, and again these graphs are cages. Thus, Moore graphs give a lower bound for the order of cages, but they are known to exist only for very specific values of k, therefore it is interesting to study how far a cage is from this bound, this value is called the excess of a cage.
We studied the excess of graphs and give a contribution, in the sense of the work of Biggs and Ito, relating the bipartition of girth 6 cages with their orders. Entire families of cages can be obtained from finite geometries, for example, the graphs of incidence of projective planes of order q a prime power, are (q+1, 6)-cages. Also by using other incidence structures such as the generalized quadrangles or generalized hexagons, it can be obtained families of cages of girths 8 and 12.
In this thesis, we present a construction of an entire family of girth 7 cages that arises from some combinatorial properties of the incidence graphs of generalized quadrangles of order (q,q)
Further topics in connectivity
Continuing the study of connectivity, initiated in §4.1 of the Handbook, we survey here some (sufficient) conditions under which a graph or digraph has a given connectivity or edge-connectivity. First, we describe results concerning maximal (vertex- or edge-) connectivity. Next, we deal with conditions for having (usually lower) bounds for the connectivity parameters. Finally, some other general connectivity measures, such as one instance of the so-called “conditional connectivity,” are considered.
For unexplained terminology concerning connectivity, see §4.1.Peer ReviewedPostprint (published version
On the connectivity of p-diamond-free vertex transitive graphs
AbstractLet G be a graph of order n(G), minimum degree δ(G) and connectivity κ(G). We call the graph G maximally connected when κ(G)=δ(G). The graph G is said to be superconnected if every minimum vertex cut isolates a vertex.For an integer p≥1, we define a p-diamond as the graph with p+2 vertices, where two adjacent vertices have exactly p common neighbors, and the graph contains no further edges. Usually, the 1-diamond is triangle and the 2-diamond is diamond. We call a graph p-diamond-free if it contains no p-diamond as a (not necessarily induced) subgraph. A graph is vertex transitive if its automorphism group acts transitively on its vertex set.In this paper, we give some sufficient conditions for vertex transitive graphs to be maximally connected. In addition, superconnected p-diamond-free (1≤p≤3) vertex transitive graphs are characterized
On several extremal problems in graph theory involving gromov hyperbolicity constant
Mención Internacional en el título de doctorIn this Thesis we study the extremal problems of maximazing and minimazing the hyperbolicity
constant on several families of graphs. In order to
properly raise our research problem, we need to introduce some important definitions and
make some remarks on the graphs we study.Programa Oficial de Doctorado en Ingeniería MatemáticaPresidente: Elena Romera Colmenarejo.- Secretario: Ana María Portilla Ferreira.- Vocal: José María Sigarreta Almir
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Extremal Directed And Mixed Graphs
We consider three problems in extremal graph theory, namely the degree/diameter problem, the degree/geodecity problem and Tur\'{a}n problems, in the context of directed and partially directed graphs.
A directed graph or mixed graph is -geodetic if there is no pair of vertices of such that there exist distinct non-backtracking walks with length in from to . The order of a -geodetic digraph with minimum out-degree is bounded below by the \emph{directed Moore bound} ; similarly the order of a -geodetic mixed graph with minimum undirected degree and minimum directed out-degree is bounded below by the \emph{mixed Moore bound}. We will be interested in networks with order exceeding the Moore bound by some small \emph{excess} .
The \emph{degree/geodecity problem} asks for the smallest possible order of a -geodetic digraph or mixed graph with given degree parameters. We prove the existence of extremal graphs, which we call \emph{geodetic cages}, and provide some bounds on their order and information on their structure.
We discuss the structure of digraphs with excess one and rule out the existence of certain digraphs with excess one. We then classify all digraphs with out-degree two and excess two, as well as all diregular digraphs with out-degree two and excess three. We also present the first known non-trivial examples of geodetic cages.
We then generalise this work to the setting of mixed graphs. First we address the question of the total regularity of mixed graphs with order close to the Moore bound and prove bounds on the order of mixed graphs that are not totally regular. In particular using spectral methods we prove a conjecture of L\'{o}pez and Miret that mixed graphs with diameter two and order one less than the Moore bound are totally regular.
Using counting arguments we then provide strong bounds on the order of totally regular -geodetic mixed graphs and use these results to derive new extremal mixed graphs.
Finally we change our focus and study the Tur\'{a}n problem of the largest possible size of a -geodetic digraph with given order. We solve this problem and also prove an exact expression for the restricted problem of the largest possible size of strongly connected -geodetic digraphs, as well as providing constructions of strongly connected -geodetic digraphs that we conjecture to be extremal for larger . We close with a discussion of some related generalised Tur\'{a}n problems for -geodetic digraphs