126 research outputs found
Subgraph conditions for Hamiltonian properties of graphs
The research that forms the basis of this thesis addresses the following general structural questions in graph theory: which fixed graph of pair of graphs do we have to forbid as an induced subgraph of an arbitrary graph G to guarantee that G has a nice structure?\ud
In this thesis the nice structural property we have been aiming for is the existence of a Hamilton cycle, i.e., a cycle containing all the vertices of the graph, or related properties like the existence of a Hamilton path, of cycles of every length, or of Hamilton paths starting at every vertex of the graph. For these structural properties, sufficient Ore-type degree conditions are known since the 1960s. These Ore-conditions are of the type: if every pair of nonadjacent vertices of the graph G has degree sum at least some lower bound, the G is guaranteed to have the structural property. In order to obtain common generalizations of these sufficiency results based on Ore-type degree sum conditions on one hand and forbidden induced subgraph conditions on the other hand, the following questions have also been addressed in the thesis. Can we restrict the corresponding Ore-type degree sum condition to certain induced subgraphs of pairs of induced subgraphs of a graph G and still guarantee that G has the same nice structure? In the thesis work we have proved many examples that provide affirmative answers to these general questions. We refer to the listed chapters for the details and the the precise definitions and formulations of the results
On path-quasar Ramsey numbers
Let and be two given graphs. The Ramsey number is
the least integer such that for every graph on vertices, either
contains a or contains a . Parsons gave a recursive
formula to determine the values of , where is a path on
vertices and is a star on vertices. In this note, we first
give an explicit formula for the path-star Ramsey numbers. Secondly, we study
the Ramsey numbers , where is a linear forest on
vertices. We determine the exact values of for the cases
and , and for the case that has no odd component.
Moreover, we give a lower bound and an upper bound for the case and has at least one odd component.Comment: 7 page
Heavy subgraphs, stability and hamiltonicity
Let be a graph. Adopting the terminology of Broersma et al. and \v{C}ada,
respectively, we say that is 2-heavy if every induced claw () of
contains two end-vertices each one has degree at least ; and
is o-heavy if every induced claw of contains two end-vertices with degree
sum at least in . In this paper, we introduce a new concept, and
say that is \emph{-c-heavy} if for a given graph and every induced
subgraph of isomorphic to and every maximal clique of ,
every non-trivial component of contains a vertex of degree at least
in . In terms of this concept, our original motivation that a
theorem of Hu in 1999 can be stated as every 2-connected 2-heavy and
-c-heavy graph is hamiltonian, where is the graph obtained from a
triangle by adding three disjoint pendant edges. In this paper, we will
characterize all connected graphs such that every 2-connected o-heavy and
-c-heavy graph is hamiltonian. Our work results in a different proof of a
stronger version of Hu's theorem. Furthermore, our main result improves or
extends several previous results.Comment: 21 pages, 6 figures, finial version for publication in Discussiones
Mathematicae Graph Theor
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